Structural aspects of the metal-insulator transitions in Cr-doped VO2

Abstract

The temperature-composition phase diagram of V1-xCrxO2 with 0≤x≤0. 025 is found to contain four phases: rutile (R), normal monoclinic VO2 (M1), and two monoclinic phases (M2 and M3). For M2 with x=0. 024 at 298 °K, 2aRaM=9. 0664, 2cRbM=5. 7970, aRcM=4. 5255, and β=91. 88. The M2 and M3 regions are separated by a volume discontinuity. There are two types of V atoms in these structures. The first type form pairs and the second type form zig-zag chains along the bM axis. This contrasts with VO2 where all the vanadium atoms are paired. The structures R, M2, and M3 with x=0. 024 have been refined from single-crystal x-ray data. High-pressure resistivity and x-ray measurements vs temperature give dT(M2→R)dP=-(0. 9±0. 1) °K/kbar and VR-VM2=0. 11 cm3/mole VO2. At higher pressures dT(M3→R)dP=(0±0. 2) °K/kbar. These results cannot be simply interpreted in terms of the homopolar bond model proposed for the M1 phase of VO2. © 1972 The American Physical Society.