Experimental Studies of Spin, Charge and Orbital Order at Extreme Conditions

Spin, charge and orbital ordering in various crystalline compounds have been studied under extreme conditions. The main techniques used were synchrotron X-ray and neutron powder diffraction. High-pressure conditions were obtained by using a diamond anvil cell and the Paris-Edinburgh cell. Changes in the valence state of BiNiO3 perovskite under pressure have been investigated by a neutron powder diffraction study and bond valence sum (BVS) calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi3+0.5Bi5+0.5Ni2+O3 with ordering of Bi3+ and Bi5+ charges on the A sites of a highly distorted perovskite structure. High pressure neutron diffraction measurements show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high pressure phase is metallic Bi3+Ni3+O3. This exceptional charge transfer between A and B site cations coupled to electronic instabilities at both sites gives rise to a remarkable variety of ground states. Furthermore, Rietveld analysis of low temperature neutron powder diffraction data shows that the structure of BiNiO3 remains triclinic (space grp 1P) throughout the temperature range 5 to 300 K. BVS calculations confirm that the charge distribution is Bi3+0.5Bi5+0.5Ni2+O3 down to 5 K. The magnetic cell is identical to that of the triclinic superstructure and a G-type antiferromagnetic model gives a good fit to the magnetic intensities, with an ordered Ni2+ moment of 1.76(3) μB at 5 K. However, BiNiO3 is ferrimagnetic due to the inexact cancellation of opposing, inequivalent moments in the low symmetry cell. The effect of high pressure on the structural properties of (EDT-TTF-CONH2)6[Re6Se8(CN)6], a conducting, molecular, mixed-valence, π-conjugated radical, cation salt has been examined using synchrotron X-ray diffraction and a diamond anvil cell set-up. It has previously been shown that this compound undergoes a low temperature phase transition from a rhombohedral (space group 3R) to a triclinic (space group1P) structure at ~150 K. This transition is caused by a charge ordering. A LeBail profile fitting of powder diffraction data revealed a change in compressibility at 0.7 GPa indicative of a phase transition. This was confirmed by single crystal data which showed that the structure remains rhombohedral )3(R, up i to 0.4 GPa but is triclinic )1(P at 0.8, 1.2 and 1.8 GPa. Hence, high pressure, as well as low temperature, can drive the charge ordering in (EDT-TTF-CONH2)6 [Re6Se8(CN)6]. The transition pressure is between 0.5-0.7 GPa at 300 K. The crystal and magnetic structures of the orbitally ordered perovskite KCrF3 have been determined from neutron powder diffraction measurement at temperatures from 3.5 to 300 K. A phase transition from a tetragonal to a monoclinic structure occurs at 250 K but the orbital ordering is sustained. Long range antiferromagnetic order of the A-type occurs below TN = 46 K and the refined magnetic moment for the Cr2+ sites was found to be 4.39(7) μB