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although pbtase@xmath0 was discovered several decades ago , @xcite its electronic structure and physical properties have only been studied in detail over the past few years . @xcite structurally , pbtase@xmath0 can be thought of as alternating stacking of hexagonal tase@xmath0 and pb layers with the @xmath9 space group . the crystal structure of pbtase@xmath0 is non - centrosymmetric . @xcite initially , only the values of resistivity and hall coefficient at 300 k of a pressed powder pellet of pbtase@xmath0 and resistive onset of superconductivity at @xmath10 k were reported . @xcite    recently pbtase@xmath0 was identified as a topological , nodal semimetal with strong spin - orbit coupling . @xcite its superconducting transition temperature in polycrystalline samples @xcite and in single crystals @xcite based on thermodynamic and transport measurements was established to be @xmath11 k. thermal conductivity and london penetration depth measurements @xcite suggested a nodeless superconducting gap structure for this material . low temperature magnetoresistance was found to be relatively high , anisotropic , and sublinear in magnetic field ; the anisotropy of the upper critical field , @xmath12 , from the resistivity measurements was reported to be temperature dependent , with the values of @xmath13 . @xcite    the complex , non - trivial band structure of pbtase@xmath0 @xcite suggests possible sensitivity of its physical properties to applied pressure . indeed , a non - monotonic , v - shaped , pressure dependence of @xmath8 was observed in a polycrystalline sample of pbtase@xmath0 that had a relatively broad resistive superconducting transition . @xcite this behavior of @xmath14 was suggested to result from a lifshitz transition under pressure . non - monotonic behavior of @xmath8 as a function of pressure has been observed in a number of materials , including elements . @xcite a lifshitz transition ( a change of the fermi surface topology ) @xcite has been invoked to explain such evolution of @xmath8 with pressure . @xcite recently , for complex superconductors , other possible causes for non - monotonic behavior of @xmath8 under pressure were discussed and studied , e.g. crossing long range magnetic order phase lines in @xmath15 phase diagrams @xcite or pressure - induced changes in superconducting pairing symmetry or gap structure . @xcite    to clarify the intrinsic @xmath14 behavior of pbtase@xmath0 under pressure and to better address the physics associated with this behavior , in this work we perform measurements of in - plane resistivity of high quality pbtase@xmath0 single crystals under pressures up to @xmath16 gpa in zero and applied magnetic field . in addition to @xmath14 , and pressure dependence of the normal state resistivity , these measurements allow us to follow the evolution of the upper critical field , @xmath17 near the @xmath18 ( that was instrumental in e.g. studies of kfe@xmath0as@xmath0 and fese @xcite ) and of the low temperature , normal state magnetoresistance . additionally , motivated by the low temperature results discussed below , ambient pressure x - ray diffraction and transmission electron microscopy measurements at elevated temperatures were performed . experimental studies were complimented by the first - principles calculations of the stability of different crystallographic phases under pressure . pbtase@xmath0 single crystals were grown by chemical vapor transport , using pbcl@xmath0 as a transport agent . more details about the synthesis are presented in ref . . the quality of the samples was attested by rather high residual resistivity ratios , @xmath19 and sharp superconducting transitions in zero field . the electrical contacts for standard 4-probe ac resistivity measurements were made using pt wires and combination of du pont 4929n silver paste and epo - tek h20e silver epoxy . the current was flowing in the @xmath20 plane and magnetic field was applied along the @xmath21-axis . the resistivity measurements at ambient and high pressure were performed in a quantum design physical property measurement system ( ppms-9 ) . for resistivity measurements under pressure a be - cu / ni - cr - al hybrid piston cylinder pressure cell similar to that used in ref . was used . a 40 : 60 mixture of light mineral oil and n - pentane was used as a pressure medium . this medium solidifies at room temperature at @xmath22 gpa @xcite , well above the pressure range used in this work . the pressure at room temperature was evaluated using manganin resistive gauge , whereas at low temperatures the superconducting transition of pure pb @xcite was used to determine pressure . additionally , low - field dc magnetization under pressure down to 1.8 k was measured in a quantum design magnetic property measurement system ( mpms-5 ) squid magnetometer using a commercial , hmd , be - cu piston - cylinder pressure cell @xcite . in these measurements daphne oil 7373 was used as a pressure medium and superconducting pb as a low - temperature pressure gauge @xcite . for magnetization measurements a stack of single crystals was oriented with @xmath23 . an additional set of resistivity measurements was performed under he - gas pressure . four - point ac electrical resistivity measurements were carried out simultaneously on two pbtase@xmath0 crystals with approximate dimensions 0.5@xmath240.1@xmath240.05 mm@xmath25 to hydrostatic pressures as high as 0.37 gpa in a he - gas high - pressure system . an excitation current of 1 ma ( rms ) at 17 hz was applied using a keithley 6221 constant ac / dc current source and the small voltage detected by a stanford research sr830 digital lock - in amplifier . experiments were carried out both at constant temperature with varying pressures or at constant pressure with varying temperatures . a janis supervaritemp cryostat was used to reach temperatures below the ambient .    to generate hydrostatic pressure the samples were placed in the 7 mm diameter bore of a be - cu high - pressure cell ( unipress , warsaw ) connected to a three - stage harwood compressor system . he gas from the compressor is fed into the pressure cell via a 3 mm od / 0.3 mm i d be - cu capillary tube . a calibrated digital manganin gauge at ambient temperature ( harwood model dj-320/42 ) accurately determines the pressure . a sizable  dead volume " at ambient temperature reduces the decrease of he - gas pressure on cooling from ambient to low temperatures . high temperature x - ray diffraction ( xrd ) was performed using a panalytical xpert pro xrd system with an anton paar htk-1200n furnace under flowing helium after evacuating and backfilling the system with he . larger single crystals were ground to a few tens of @xmath26 m but retained their micaceous morphology hence the xrd only exhibited ( 00l ) reflections . copper and cobalt radiation was used . heating rates were either 3 k / min or 5 k / min . since the xrd was not able to resolve changes of the _ in - plane _ lattice with heating , additional studies using transmission electron microscopy ( tem ) at different temperatures using a fei tecnai g2 f200 instrument operating at 200 kv were performed . for these studies a single crystal was thinned via ar ion milling . the thin region was co - planar with the basal planes , providing an orthogonal view of the lattice expansion on heating compared to the xrd results , i.e. , the ( hk0 ) reflections . the in situ heating / cooling was performed on a gatan heating stage up to @xmath27 k. continuous recording of the selected area diffraction was obtained on cooling .      the first - principles density functional theory ( dft ) @xcite calculations were performed using the vienna ab - initio simulation package ( vasp ) @xcite with projector - augmented wave ( paw ) pseudopotential method @xcite and plane wave basis . the generalized - gradient approximation parameterized for solids ( pbesol ) @xcite was used for the exchange - correlation energy functional . spin - orbit coupling was included in the calculations . the energy cutoff is 320 ev and the monkhorst - pack s scheme @xcite was used for brillouin zone sampling with a @xmath28-point mesh of @xmath29 for the ground state @xmath9 structure and equivalent @xmath28-point meshes for other structures . all crystal structures were fully relaxed until the forces on each atom were smaller than 0.01 ev /   and external pressure was smaller than 0.1 gpa . ambient pressure resistivity data ( fig . [ f1]a ) are grossly consistent with those reported in refs . . although there is a region of @xmath30 behavior in resistivity , it does not persist to the temperatures close to the superconducting transition , where the behavior with temperature to higher power ( close to 4 ) was observed ( fig . [ f1]b ) . the upper critical field , @xmath31 , was determined from electrical transport and magnetization measurements ( see appendix a for details ) . here , as well as in the literature , @xcite there is an apparent discrepancy between the results of thermodynamic and transport measurements .    turning to superconducting properties under pressure , the evolution of the superconducting transition under pressure , as measured by resistivity and low field magnetization , is presented in fig . [ f3 ] . in the resistivity measurements the transitions at all pressures , other than 0.24 gpa , are sharp . from both measurements it appears that the @xmath14 has a step - like behavior . indeed , as it is seen in fig . [ f4 ] , both measurements result in consistent data , and a clear , sharp step in @xmath14 is observed at about 0.25 gpa . the broad , two - step - like resistive transition at 0.24 gpa corresponds to this apparent phase boundary . normal state resistivity at 5 k , just like @xmath8 , has a step - like behavior ( fig . the initial slope of transport @xmath31 ( see appendix b ) has a step - like change at @xmath32 gpa as well . furthermore , the normal state properties show discontinuities at @xmath33 gpa . the field dependent magnetoresistance data ( fig . [ f7 ] ) clearly fall on two different manifolds , for @xmath34 gpa and @xmath35 gpa , that have different functional dependencies of @xmath36 . a careful look at zero field , temperature - dependent resistivity data taken in the piston - cylinder cell at small , from 0.05 to 0.39 gpa at room temperature , pressures ( fig . [ f9 ] ) reveals a clear , hysteretic in temperature , sharp transition . although qualitatively these data point to an additional phase line , probably of the structural transition , that has a very steep pressure dependence , a quantitative analysis of these data is hindered by an experimental issue associated with a use of piston - cylinder pressure cells over an extended temperature range . due to differential thermal contraction of the materials of the cell and the medium , the pressure inside the cell decreases on cooling @xcite . this pressure drop depends on multiple factors , including the cell materials and design , medium and the pressure range . at low pressures , even when using manganin as a room temperature pressure gauge and pb as a low temperature pressure gauge , the evaluation of pressure at intermediate temperatures has substantial error bars .    to address the pressure dependence of the apparent structural transition in a quantitative manner , a set of measurements in a he gas system was performed ( fig . [ f10 ] ) . the hysteretic nature of the transition is seen both in temperature sweeps and pressure sweeps . these signatures are sharp and well defined . from the pressure sweeps ( fig . [ f10](b ) ) it is seen that ( a ) at lower temperatures the transition shifts to higher pressures ; ( b ) as has been seen in figure [ f9 ] , the size of the resistance jump becomes smaller at lower temperatures and it appears not to be detected anymore in the pressure sweep at @xmath37 k. fig . [ f11 ] shows that both absolute and relative resistance jump decrease at lower temperatures , and it is not clear if at 50 k the jump disappears , or just becomes on the level of the noise in the data . the pressure dependence of the apparent structural transition measured in gas pressure system is presented in fig . the second order polynomial fits to the @xmath38 data extrapolate to 0.25 gpa for the decreasing temperature manifold to intercept the @xmath39 k axis . in a similar manner , both manifolds extrapolate to @xmath2 k at ambient pressure . this is a rather rough extrapolation , additional , preferably structural , data at high temperatures are required to verify the nature of the transition .    in situ xrd was performed at different elevated temperatures . as mentioned above , the pbtase@xmath0 powder retained the micaceous morphology , so only ( 00l ) reflections could be detected and followed as a function of temperature . the experimental results are shown in fig . the ( 00l ) reflections show a clear , step - like , contraction in the @xmath21-axis lattice on heating which is reversible upon cooling . the temperature of this transition is @xmath40 k.    to resolve changes of the in - plane lattice with heating , tem measurements were performed ( fig . [ f16 ] ) . the thin region studied with tem was co - planar with the basal planes , providing an orthogonal view of the lattice expansion on heating compared to the xrd results , i.e. , the ( hk0 ) reflections . continuous recording of the selected area diffraction was obtained on cooling . the sample was slowly cooled over the temperature range of @xmath41 k to @xmath42 k at @xmath43 k / min . an abrupt contraction of the basal plane lattice of @xmath44 occurred @xmath2 k. no other changes in the diffraction pattern were observed , indicating that like in the xrd there is a discrete change in the cell parameters occurring at @xmath2 to 430 k but no obvious change in the space group ( fig . [ f16 ] , lower panels ) . tem results suggest that the basal plane undergoes a normal expansion with heating and contraction on cooling in contrast to the xrd results which shows a large contraction in the @xmath21-axis at @xmath2 k.    aberration corrected scanning transmission electron microscopy using a fei titan themis 300 cubed 300 stem / tem shows that the room temperature atomic decoration is fully consistent with the space group # 187 , @xmath9 , ( fig . [ f17 ] ) . the image suggest minimal chemical disorder . there is a rather large gap between the pb layers relative to the ta / se inner layers which form an open network of edge sharing prisms . this large gap between these layers maybe responsible for the lattice contraction in the @xmath21-axis with heating .    altogether on heating through the structural transition at @xmath45 k the @xmath46-axis increases by @xmath47 , whereas the @xmath21-axis decreases by @xmath48 , leading to a decrease of the unit cell volume by @xmath49 . based on the data discussed above , the pressure - temperature the phase diagram for pbtase@xmath0 ( fig . [ f13 ] ) appears to have two superconducting phases with the boundary between them defined by a structural phase transition that has extremely steep pressure dependence . since the normal state resistivity , magnetoresistance and the initial slopes of @xmath50 are different in these two phases , clearly , the electronic structure is affected . a bloch - grneisen fit of resistivity ( over the 50 k - 300 k temperature range ) ( fig . [ f14 ] ) . suggests that as a result of the structural phase transition the debye temperature increases , so that the lattice becomes stiffer . this is largely consistent with an overall decrease of the unit cell volume at ambient pressure on heating through the transition . despite this , @xmath8 is lower in the new structural phase , suggesting that either the change in the electronic subsystem is the dominant contribution to the @xmath8 decrease , or that the in - plane phonons are more important for superconductivity than the out - of - plane phonons . the latter hypothesis is in agreement with the observed large gaps between the layers ( fig . [ f17 ] ) .    since available experimental techniques do not allow for an unambiguous identification of the high pressure ( high temperature ) phase , we have performed first - principles calculations that address relative stability of several related hexagonal phases under pressure . in addition to the known @xmath9 structure of pbtase@xmath0 , we consider 3 other low - energy structures ( within 50.0 mev / atom with respect to that of the @xmath9 structure ) coming from our crystal structure optimization scheme with one and two formula units and hexagonal symmetry constraints ( fig . [ f18 ] ) . these structures can also be obtained through modification of the @xmath9 structure . the @xmath51 structure can be obtained from the @xmath9 structure by moving the pb atom from the @xmath3-wyckoff position to the @xmath52-wyckoff position . similarly the @xmath53 structure can be obtained by shifting the pb from the @xmath3-wyckoff position to the @xmath4-wyckoff position . by doubling unit cell of the @xmath9 structure along the @xmath21-axis lattice vector and then moving the upper half of unit cell by 1/3 along the long diagonal of the basal plane @xmath54 , the @xmath55 structure can be obtained . the @xmath51 and @xmath53 structures are in @xmath9 space group symmetry whereas the @xmath55 structure is in @xmath56 space group symmetry . the lattice parameters and wyckoff positions of these structures are given in table [ t1 ] . the lateral lattice constants of these 3 structures are similar to that of the @xmath9 structure , but their lattice constants along the @xmath21 direction ( @xmath57 for the @xmath55 structure ) are smaller than that of the @xmath9 structure by 0.534  , 0.487  , and 0.246   respectively , where the @xmath51 structure has the shortest lattice parameter @xmath21 .    in order to compare with experimental xrd results , we simulated the xrd ( 0 0 l)-peaks of all four structures considered . it is interesting to note that although the @xmath55 structure has a doubled unit cell along the @xmath21-direction in comparison with other structures , it shows only ( 0 0 l)-peaks with even - l . as shown in fig . [ f19 ] , the xrd ( 0 0 l)-peaks versus @xmath58-spacing patterns from all four structures are very similar . in figure [ f19 ] the peak indexes of the @xmath55 structure are ( 0 0 2l ) of the labeled ones and the diffraction intensities of all structures are normalized by the intensity of their ( 0 0 2)-peak . figure [ f20 ] shows the relative formation enthalpies of different pbtase@xmath0 structures as a function of pressure with respect to that of the @xmath9 structure . at zero and low pressure , the stable structure is the @xmath9 structure . as the pressure is increased , the @xmath53 structure becomes more stable with a structural transition from the @xmath9 to the @xmath53 at 3 gpa . at ambient conditions , the formation energy of the @xmath53 structure is 25.9 mev / atom higher than that of the @xmath9 ground state structure . this energy difference is about the room temperature thermal energy . we note that the @xmath55 structure is lower in energy than the @xmath53 structure at ambient conditions . however , the @xmath53 structure becomes more stable than the @xmath55 structure when the external pressure is greater than 2.7 gpa , which is just below the transition pressure from the @xmath9 structure to the @xmath53 structure . thus these calculations suggest that the experimentally observed high pressure ( high temperature ) structure is the @xmath53 structure . we would like to note that structural transition pressure from the dft calculations is higher than that observed in experiment . this discrepancy is probably due to the systematic error in dft calculation of pressure . for example , in the literature the predicted structural transition pressure of si from cubic diamond ( si - i ) to - sn ( si - ii ) phase can be several gpa off the experimentally observed value , depending on exchange - correlation functional used.@xcite but the sequence of phase transition , i.e. si - i to si - ii to si - v to si - vi to si - vii , from dft calculation is consistent with experiment . @xcite + _ to summarize _ , pbtase@xmath0 exhibits a structural , sharp , first order , phase transition at a very moderate pressure of @xmath1 gpa at low temperatures . pbtase@xmath0 has a @xmath59 k / gpa making it one of the more pressure sensitive transitions found in inorganic compounds . the structural phase transition line extends to @xmath2 k at ambient pressure as evidenced by transmission electron microscopy and x - ray diffraction at elevated temperatures . upon transition to the new phase ( on increase of temperature at ambient pressure ) the @xmath21-axis decreases and the @xmath46-axis increases , resulting in a slight , @xmath49 decrease of the unit cell volume . the new , high temperature / high pressure phase has similar crystal structure . as suggested by the first - principles calculations , in this structure pb shifts from @xmath3 to @xmath4 wyckoff position with ta and se positions remaining the same . the superconductivity appears to be robust , it persists through the structural phase transition into the other phase with slight , step - like decrease of @xmath8 . ambient pressure , low temperature resistivity , @xmath60 , measured in constant fields and @xmath61 , measured at constant temperatures , are shown in figs . [ f2]a , b . two criteria , onset and offset ( see fig . [ f2]b ) were used to evaluate the @xmath12 . additionally , zero - field - cooled magnetization was measured in different applied fields ( fig . [ f2]c ) and @xmath31 was determined from these measurements as well . the summary of these results , the ambient pressure @xmath31 for @xmath23 inferred from electrical transport and magnetization measurements are shown in fig . [ f2]d and are compared to the literature data @xcite . the resistively determined @xmath31 is comparable with the literature data ( with possible contributions to the difference from the sample shape and slight mis - orientation ) . similarly , results from magnetization are very close to the published upper critical field determined from specific heat . however , both in the literature @xcite and in this work , even though the low field / zero field @xmath8 values are quite similar , @xmath31 from electrical transport [ @xmath62 and thermodynamic [ @xmath63 measurements are noticeably different . the origin of this difference is not clear at this point . the temperature - dependent upper critical field for @xmath23 was measured resistively as a function of pressure ( fig . [ f5 ] ) . whereas there is an apparent difference between thermodynamic and transport @xmath12 values ( see above ) , as well as an upward curvature of @xmath31 and the limited range of the data for the higher pressures , we can still compare the change of the initial , close to @xmath64 , slope of transport @xmath12 as a function of pressure ( fig . [ f6 ] ) . in agreement with the @xmath14 behavior , both bare , @xmath65 , and normalized , @xmath66 , initial slopes of the upper critical field have step - like change at @xmath32 gpa . we thank the ms . junying shen for assistance in the he - gas measurements . work at the ames laboratory was supported by the u.s . department of energy , office of science , basic energy sciences , materials sciences and engineering division . the ames laboratory is operated for the u.s . department of energy by iowa state university under contract no . de - ac02 - 07ch11358 , including a grant of computer time at the national energy research scientific computing centre ( nersc ) in berkeley , ca . the work at washington university was supported by the national science foundation ( nsf ) through grants no . dmr-1104742 and 1505345 . guang bian , tay - rong chang , raman sankar , su - yang xu , hao zheng , titus neupert , ching - kai chiu , shin - ming huang , guoqing chang , ilya belopolski , daniel s. sanchez , madhab neupane , nasser alidoust , chang liu , baokai wang , chi - cheng lee , horng - tay jeng , chenglong zhang , zhujun yuan , shuang jia , arun bansil , fangcheng chou , hsin lin , and m. zahid hasan , nat . * 7 * , 10556 ( 2016 ) . e. hassinger , g. gredat , f. valade , s. ren de cotret , a. juneau - fecteau , j .- ph . reid , h. kim , m. a. tanatar , r. prozorov , b. shen , h .- h . wen , n. doiron - leyraud , and louis taillefer , phys . b * 86 * , 140502 ( 2012 ) . valentin taufour , neda foroozani , makariy a. tanatar , jinhyuk lim , udhara kaluarachchi , stella k. kim , yong liu , thomas a. lograsso , vladimir g. kogan , ruslan prozorov , sergey l. budko , james s. schilling , and paul c. canfield , phys . rev . b * 89 * , 220509 ( 2014 ) .                                   . inset : low temperature part of the data showing the superconducting transition . ( b ) low temperature resistivity of pbtase@xmath0 at ambient pressure plotted as @xmath67 vs @xmath68 . the dashed line is a guide for the eye . inset : _ log - log _ plot of the low temperature resistivity after subtraction of the residual resistivity @xmath69 . lines show @xmath68 and @xmath70 behavior . , width=453 ]     correspond to the pressure runs that have slightly different room temperature pressure values but result in the same @xmath71 low temperature value as measured by pb manometer . for magnetization measurements a deviation from linear , normal state magnetization ( indicated by circle ) was taken as a @xmath8 criterion , see inset to the panel ( b ) . , title="fig:",width=377 ]   correspond to the pressure runs that have slightly different room temperature pressure values but result in the same @xmath71 low temperature value as measured by pb manometer . for magnetization measurements a deviation from linear , normal state magnetization ( indicated by circle ) was taken as a @xmath8 criterion , see inset to the panel ( b ) . , title="fig:",width=377 ]     as determined from resistivity and magnetization measurements ( left y - axis ) , and normal state resistivity at 5 k ( right y - axis ) as a function of pressure . dashed vertical line shows @xmath72 gpa location . other dashed lines are guides for the eye.,width=453 ]     k , field dependent magnetoresistance at different pressures ( listed in the upper panel in units of gpa ) , plotted as @xmath61 and @xmath73 . note : for both panels the four p = 0 data sets , as well as the @xmath74 gpa data set form an upper manifold , and the @xmath35 gpa data sets form a lower manifold.,width=453 ]                                     data taken in applied magnetic fields from @xmath77 to @xmath78 koe with 0.1 koe steps . ( b ) field dependent resistivity in fixed magnetic fields . onset and offset criteria are shown for 3 k dataset . ( c ) low temperature zero - field - cooled magnetization data taken in different magnetic fields . arrows mark the superconducting transition temperatures . ( d ) zero pressure @xmath31 data for @xmath23 obtained from resistivity ( onset and offset criteria ) - red circles @xmath61 , black circles @xmath60 , and magnetic susceptibility data - stars , plotted together with the literature data @xcite from resistivity ( triangles ) and heat capacity ( rhombi ) measurements in magnetic field.,title="fig:",width=302 ]    data taken in applied magnetic fields from @xmath77 to @xmath78 koe with 0.1 koe steps . ( b ) field dependent resistivity in fixed magnetic fields . onset and offset criteria are shown for 3 k dataset . ( c ) low temperature zero - field - cooled magnetization data taken in different magnetic fields . arrows mark the superconducting transition temperatures . ( d ) zero pressure @xmath31 data for @xmath23 obtained from resistivity ( onset and offset criteria ) - red circles @xmath61 , black circles @xmath60 , and magnetic susceptibility data - stars , plotted together with the literature data @xcite from resistivity ( triangles ) and heat capacity ( rhombi ) measurements in magnetic field.,title="fig:",width=302 ]    data taken in applied magnetic fields from @xmath77 to @xmath78 koe with 0.1 koe steps . ( b ) field dependent resistivity in fixed magnetic fields . onset and offset criteria are shown for 3 k dataset . ( c ) low temperature zero - field - cooled magnetization data taken in different magnetic fields . arrows mark the superconducting transition temperatures . ( d ) zero pressure @xmath31 data for @xmath23 obtained from resistivity ( onset and offset criteria ) - red circles @xmath61 , black circles @xmath60 , and magnetic susceptibility data - stars , plotted together with the literature data @xcite from resistivity ( triangles ) and heat capacity ( rhombi ) measurements in magnetic field.,title="fig:",width=302 ]    data taken in applied magnetic fields from @xmath77 to @xmath78 koe with 0.1 koe steps . ( b ) field dependent resistivity in fixed magnetic fields . onset and offset criteria are shown for 3 k dataset . ( c ) low temperature zero - field - cooled magnetization data taken in different magnetic fields . arrows mark the superconducting transition temperatures . ( d ) zero pressure @xmath31 data for @xmath23 obtained from resistivity ( onset and offset criteria ) - red circles @xmath61 , black circles @xmath60 , and magnetic susceptibility data - stars , plotted together with the literature data @xcite from resistivity ( triangles ) and heat capacity ( rhombi ) measurements in magnetic field.,title="fig:",width=302 ]
transport and magnetic studies of pbtase@xmath0 under pressure suggest existence of two superconducting phases with the low temperature phase boundary at @xmath1 gpa that is defined by a very sharp , first order , phase transition . the first order phase transition line can be followed via pressure dependent resistivity measurements , and is found to be near 0.12 gpa near room temperature . 
 transmission electron microscopy and x - ray diffraction at elevated temperatures confirm that this first order phase transition is structural and occurs at ambient pressure near @xmath2 k. the new , high temperature / high pressure phase has a similar crystal structure and slightly lower unit cell volume relative to the ambient pressure , room temperature structure . 
 based on first - principles calculations this structure is suggested to be obtained by shifting the pb atoms from the @xmath3 to @xmath4 wyckoff position without changing the positions of ta and se atoms . 
 pbtase@xmath0 has an exceptionally pressure sensitive , structural phase transition with @xmath5 k / gpa near 4 k , this first order transition causes an @xmath6 k ( @xmath7 ) step - like decrease in @xmath8 as pressure is increased through 0.25 gpa .