Transition between Orbital Orderings inYVO3

Abstract

Evidence has been found for a change in the ordered occupation of the vanadium d-orbitals at the 77K phase transition in YVO3, manifested by a change in the type of Jahn-Teller distortion. The orbital ordering above 77K is not destroyed at the magnetic ordering temperature of 116K, but is present as far as a second structural phase transition at 200K. The transition between orbital orderings is caused by an increase in octahedral tilting with decreasing temperature. [5,6].In this letter we report on the orbital ordering in YVO 3 . The orbital ordering of t 2g electron systems is reflected structurally to a lesser extent than that of e g systems but can still have a dramatic influence on the physical properties. We show that the orbital ordering in YVO 3 takes place at 200K which, contrary to previous assumptions, is far above the antiferromagnetic (AFM) ordering temperature T N = 116K. Moreover, there is a change in symmetry of the orbital ordering at T S = 77K. This is induced by an increased tilting of the octahedra with decreasing temperature, and it changes the easy axis of the V 3+ d 2 S = 1 spin, resulting in a magnetic structure transition and a reversal of the net ferromagnetic moment of the canted AFM state [7].We have previously described the unusual magnetic properties of YVO 3 [7,8]. YVO 3 adopts a distorted perovskite structure with the space group P bnm [9] at room temperature. At T S there is a first order structural phase transition involving a sudden change in the unit cell volume, below which a JT ordered state is present [8]. A tetragonal distortion of the octahedra, where the long and short V-O bond distances alternate along the [110] and [110] directions of the ab plane, causes a splitting of the V t 2g orbitals into a doublet of lower energy and a singlet of higher energy. The doublet contains the xy orbital, which is always occupied, and either the zx or yz orbital. This ordered, alternating occupation of the V d zx and d yz orbitals between adjacent cations is shown schematically in Figs. 1 and 2. We believe that the change in magnetic structure [10] is caused by a change from C-type orbital ordering (OO) below T S to G-type OO above T S (Fig. 2). This statement is supported by the Goodenough-Kanamori rules [11], band structure calculations [12] and model Hartree-Fock calculations [13]. Experimentally, the only suggestion of G-type OO thus far has been provided by resonant X-ray scattering studies at the vanadium main K edge [14], although there is some debate about whether orbital ordering can be directly observed at this energy [15,16]. In terms of the crystal structure, G-type OO is incompatible with P bnm symmetry since two crystallographically distinct JT-distorted ab planes are required, with "out of phase" bonding arrangements; in P bnm all ab planes are rendered equivalent by the mirror planes at z = 1/4 and 3/4, and the bonding arrangement is "in phase". We provide here the first crystallographic evidence for G-type OO above T S .Single crystals of YVO 3 were prepared as prev...

Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights

“…These classical states are frequently assumed as a consequence of a strong JT term and observed GdFeO 3 distortions. 94 Such classical order would naturally follow from a strong JT effect, but it is still controversial whether the JT interaction is actually that strong in the vanadates. Fortunately, there are already experimental results which can help to resolve this controversy, and we address this issue in more detail in Sec.…”

“…A general Hamiltonian was already given in Ref. 16; here we shall analyze it assuming that the xy͉͑c͒͘ orbitals are singly occupied at each V 3+ ion, as concluded from experiment 93,94 and from electronic structure calculations. 95 Therefore, the electron densities in the remaining two orbitals satisfy at each site i the local constraint n ia + n ib = 1.…”

“…In the next Sec. VI B we will present now the limitations of a simplified approach which is widely accepted 94 and assumes that the OO in LaVO 3 is quite rigid already at the magnetic transition. Quantum fluctuations lead to important corrections which go beyond this picture and modify the temperature variation of the optical spectral weights, as we discuss in Sec.…”

Y1−xLaxVO3: Effects of doping on orbital ordering

“…The transitions in the orbital and magnetic degrees of freedom are coupled with structural transitions. Above 200 K and below 77 K, the crystal symmetry is orthorhombic, while at intermediate temperatures, 77 K < T < 200 K, the symmetry is monoclinic [23]. For comparison, on the other end, in LaVO 3 , the T OO transition is suppressed, while the antiferromagnetic transition at T N ∼ 143 K is quickly followed by a structural transition (T t ∼140 K) from the orthorhombic to the monoclinic phase, the latter of which accommodates a G-OO/C-SO phase [24].…”

“…The orbital patterns in the two symmetries are shown in Figs. 1(a) and 1(b) [23], where in (a) the orbitals are in-phase along the c-axis in the Pbnm symmetry while in (b), the orbitals are out-ofphase in the P2 1 /a symmetry. Also shown in this figure is the spin orientation of Ti 3+ ions.…”