Crystal structures of heaviest nuclides' compounds
Since there were no further suggestions, I decided to use the references from my comment in an answer, which, I believe, is element 99, einsteinium.

Both single crystal x-ray diffraction (SCXRD) and powder x-ray diffraction (PXRD) have certain sample requirements/limitations:


Mass/amount


PXRD: lower limit is in micrograms region, ideally at least a couple of milligrams of polycrystalline sample;
SCXRD: a single crystal of a size of fractions of a millimeter.

Time


PXRD: solid polycrystalline sample must be homogeneous which sometimes requires prolonged synthesis time (minutes, hours) due to diffusion barrier (solid-solid, solid-liquid or solid-gas). A measurement itself on average only takes minutes;
SCXRD: in addition to the synthesis, growing a suitable single crystal for analysis normally takes days, maybe hours if you are lucky. A measurement itself on average takes hours, sometimes days.



Looking both at the end of periodic table of elements and these two constrains, I suspect it is unlikely that current techniques can be applied for transfermium elements and fermium itself as we neither have the necessary amounts nor time.

Among transuranium elements with $Z &lt; 100$ plutonium $\ce{_{94}Pu}$ compounds appear to be of the greatest interest and probably the greatest number of published structures, too.
However, einsteinium $\ce{_{99}Es}$ exists as several relatively stable isotopes with half-lives of the magnitude of months and days and has a somewhat developed chemistry to speak of.

In 1968 Fujita et al. [1] having 3 μg of $\ce{^{253}Es}$ synthesized and reported crystal structures of einsteinium(III) chloride $\ce{EsCl3}$ and einsteinium(III) oxochloride $\ce{EsOCl},$ both determined via powder x-ray diffraction.
Reading the full text is encouraged, mainly to admire authors' ingenuity in overcoming technical difficulties (self-contamination of the isotope, film darkening by radiation, measurement of lattice parameters in temperature range from the room temperature to over 400 °C and so on) and overall concise writing style of the report.

$\ce{Es^3+}$ in both structures has C.N. 9 and coordinates in a single-capped square antiprism ligand environment:



Figure 1. Crystal structure of $\ce{EsCl3}$ (ICSD-109002). Color code: $\color{#1FF01F}{\Large\bullet}~\ce{Cl}$; $\color{#B31FD4}{\Large\bullet}~\ce{Es}$.



Figure 2. Crystal structure of $\ce{EsOCl}$ (ICSD-109003). Color code: $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#1FF01F}{\Large\bullet}~\ce{Cl}$; $\color{#B31FD4}{\Large\bullet}~\ce{Es}$.

A few years later Haire and Baybarz [2] determined crystal structure of einsteinium(III) oxide $\ce{Es2O3}$ via electron diffraction, but this is another story.

References


Fujita, D. K.; Cunningham, B. B.; Parsons, T. C. Crystal Structures and Lattice Parameters of Einsteinium Trichloride and Einsteinium Oxychloride. Inorganic and Nuclear Chemistry Letters 1969, 5 (4), 307–313. https://doi.org/10/bxzkjh. (PDF)  
Haire, R. G.; Baybarz, R. D. Identification and Analysis of Einsteinium Sesquioxide by Electron Diffraction. Journal of Inorganic and Nuclear Chemistry 1973, 35 (2), 489–496. https://doi.org/10/bbfgp4.