Let $f\colon X \to S$ be a smooth proper morphism of schemes. If $S$ is of characteristic zero (i.e., $S$ is a $\mathbb Q$-scheme), then Deligne has shown:


1. $R^af\_\*\Omega^b\_{X/S}$ is locally free for all $a,b \geq 0$.
2. The Hodge-De Rham spectral sequence
$E^{ab}\_1 = R^af\_\*\Omega^b(X/S) \Rightarrow H\_{\rm DR}^{a+b}(X/S)$
degenerates in $E\_1$.


This is known to fail in positive characteristic. Mumford gave examples of smooth projective surfaces over algebraically closed fields. Nevertheless there are several interesting cases of schemes $X \to S$ in characteristic $p > 0$ where I know this to be true:


a. $X$ is an abelian scheme, a relative curve, a global complete intersection in projective space, or a K3-surface over $S$.


b. $X$ is a smooth projective toric variety over a field.


c. There is also a criterion of Deligne and Illusie which in particular shows 1. und 2. to hold if $\dim(X/S) < p$ and $X$ can be lifted to $W\_2(S)$.


**Question**: What are other examples in positive characteristic, where 1. and 2. hold?


There is also a variant of the result of Deligne for logarithmic schemes. In particular I would be also interested for examples where the logarithmic analogue of 1. and 2. hold.


ADDITION: I am taking the risk to name two examples of smooth projective schemes over a field, where I would not be too surprised if (1. and) 2. hold, but where I know of no results:


d. $X$ is Calabi-Yau (i.e., its canonical bundle is trivial).


Edit: As Torsten Ekedahl pointed out below this definition of "Calabi-Yau" is not the "right" one (not even in char. $> 2$ as I wrote in an earlier edit) and does not imply in general that the Hodge-De Rham spectral sequence degenerates.


e. $X$ is $G$-spherical for a reductive group $G$ (i.e., $X$ carries a $G$-action such that there exists a dense $B$-orbit, where $B$ is a Borel subgroup of $G$).


Edit: Again one might to have exclude some small primes depending on the Dynkin type of $G$.



Degeneration of the Hodge spectral sequence
[I misunderstood Torsten Ekedahl's earlier comment. I'm reverting the lemma
to its original form which was a bit stronger.]


Since the question seemed to resonate with me, I've been thinking about this on and off (but mostly off) for a couple of days now. Here's what I've come up with.


What seems to make the example of complete intersections work is the fact that
the Hodge numbers can be computed by formulas independent of the characteristic
(a standard generating function can be found in SGA7, exp XI).
Here's the underlying principle.



> 
> Lemma.
>  Suppose that $D$ is the spectrum of a mixed characteristic DVR with closed point $0$ and
>  generic point $\eta$. Let $\mathcal{X}\to D$ be a smooth projective family such that
>  $$\dim H^q(\mathcal{X}\_0,\Omega\_{\mathcal X\_0}^p)= \dim H^q(\mathcal{X}\_\eta,\Omega\_{\mathcal X\_\eta}^p)$$
>  for all $p,q$. Then Hodge to De Rham degenerates on the closed fibre.
> 
> 
> 


Proof. It degenerates on $\mathcal{X\_\eta}$ by Hodge theory. This plus semicontinuity
implies
$$\dim E\_1(\mathcal{X}\_0)\ge \dim E\_\infty(\mathcal{X}\_0)\ge \dim E\_\infty(\mathcal{X}\_\eta)
=\dim E\_1(\mathcal{X}\_\eta)=\dim E\_1(\mathcal{X}\_0)$$


This can be used to check degeneration for the following cases:


Ex 1. Complete intersections in projective spaces as noted already. 


Ex 2. Products of smooth projective curves and complete intersections. Use
Kuenneth and the fact that curves lift into characteristic $0$ (the obstruction
lies in $H^2$ of the tangent sheaf; or see Oort, Compositio 1971).


Ex 3. Certain cyclic branched covers of projective space, and more generally
certain hypersurfaces in weighted projective space. I'm too lazy to say
what "certain" means exactly. But a careful reading of Dolgachev's notes
on weighted projective spaces ought to yield something more precise.


