A plethora of structural transitions, distortions and modulations in Cu-doped BiMn7O12 quadruple perovskites

The presence of strongly competing electronic instabilities in a crystalline material can produce fascinating structural phenomena. For example, the A-site-ordered quadruple perovskite BiMn7O12 hosts both active polar instabilities of the Bi3+ lone pair electrons and Jahn-Teller instabilities of Mn3+ cations that drive the following sequence of phase transformations on cooling, Im-3 > I2/m > Im > P1, corresponding to orbital ordering and polar distortions. Carrier doping by Cu2+ tunes the two instabilities in BiCuxMn7-xO12 solid solutions and significantly complicates the system behavior. The x = 0.05 and 0.1 members show the following sequence of phase transformations on cooling, Im-3 > I2/m > R-1(alpha beta gamma)0 > R3(00 gamma)t, and are examples of materials with the electric dipole helicoidal texture in the ground state and a dipole density wave structure in the intermediate R-1(alpha beta gamma)0 phase (Science 2020, 369, 680-684). Here, the detailed behavior of the BiCuxMn7-xO12 solid solutions with x = 0.2-0.8 was investigated by laboratory X-ray, synchrotron X-ray, and neutron powder diffraction between 5 K and 620 K, and differential scanning calorimetry measurements. Nearly every composition (with a step Delta x = 0.1) has a unique behavior when considering both the sequence of phase transitions and the presence of incommensurate superstructure reflections. The sequence Im-3 > HT-Immm(t)* > Immm* > LT-Immm(t)* is realized for x = 0.2 and 0.3 (where t denotes pseudo-tetragonal), Im-3 > I2/m* > Immm(t)* - for x = 0.4, Im-3 > I2/m* > I2/m* - for x = 0.5, Im-3 > I2/m* > Im-3 - for x = 0.6 and 0.7, and Im-3 > R-3 > I2/m > Im-3 - for x = 0.8, where asterisks denote the presence of additional incommensurate reflections. Re-entrance of the high-temperature cubic phase was observed at low temperatures for x = 0.6-0.8 suggesting strong competition between the different electronic instabilities. The re-entrant cubic phases have nearly zero thermal expansion.