STRUCTURAL STUDIES OF ZRV2-XPXO7 SOLID-SOLUTIONS USING P-31-(V-51) AND V-51-(P-31) ROTATIONAL ECHO DOUBLE-RESONANCE NMR

The negative thermal expansion coefficients of ZrV2-xPxO7 solid soluti ons are attributed to the structural flexibility of M-O-M' dimers (M, M' = P, V), upon which the structure of these materials is based. The local environments of these M species have been probed by high-resolut ion P-31 and V-51 solid state NMR experiments, assisted by V-51-{P-31} and P-31-{V-51} rotational echo double resonance (REDOR) studies for assignment purposes. At all of the compositions studied, formation of mixed P-O-V dimers is distinctly preferred over the symmetric P-O-P an d V-O-V species, indicative of chemical ordering. Analysis of the REDO R experiments, assuming simple spin 1/2 behavior of the V-51 nuclei, y ields an estimate of the P ... V internuclear distance in the mixed di mers of 3.42 Angstrom, consistent with a linear geometry of the P-O-V units as required by the space group symmetry. This result is subject to the important caveat that a precise theoretical description of REDO R behavior involving quadrupolar nuclei is not yet available.