M. Greenblatt Rutgers University New Brunswick, NJ 08903
Electron Spin Resonance of Tetrahedral Transition Metal Oxyanions (M04 "-) in Solids
Electron spin resonance (ESR) is observable only in sub stances that contain a sufficient number of oermanent maanetic dipoles. Cumpound, 01 the transition metal ions w i t h unpaired electrons are paramagnetic nnd ESR ha< been used extensively tu investigaw the elt.ctronic struclllre of tranailion metal ims. Several exct4lent text hoa~ksI I ) and review articles (2) deal with this subject in detail. Generally, to observe sharp ESR lines in the solid state it is necessary t o i n ~ o r p ~ r athe t e transition metal i m of intrreht into nn isustrucruraldismngnetich g~ and IAl < IBI (A = A. a n d B = (A, Ay)/2),while for ad,, orbital g, is generally greater than gll and IAl > IBI. These general features will remain true even in cases of lower symmetry wheng, # g, and A, # A, by approximatingg, -g, = g , andA,-A, =A,. The experimental results in Tahle 1show gl > g l , and ]A1 < IBI indicating a d,z ground state for Cr5+ in all of the host crystals except in KH2P0g and NaVOz. From crystal field theory, with compression of the tetrahedron along one of its S g symmetry axes, such that two 0-Cr-0 angles will he larger and four smaller than the ideal 109.5" tetrahedral angle, the d,z orhital of a d' configuration will lie lowest and with elongation along the Sq axis (two 0 - C r - 0 angles smaller and four larger than 109.5") the d,, orhital is lowest. It is shown that in KH7POa despite the .(and in KH?AsOd), . Cr04" tetrahedron being "squashed," g < g l , and-the d,, eround state orhital is stabilized due to the interaction of the electron with the positive charges of four nearby protons (22). For Cr5+ in YVOg and YPOg the tetrahedra are strongly distorted to Dzd symmetry by a stretching motion along the c crystal axis, predicting ad,, ground state for the d' electron in these hosts. However, gll> g L and I A I < I B I are observed, which are characteristic of a d,z ground state. The inversion appears to he due to the interaction of the electron on chromium with two neighboring yttrium ions via the commonly shared oxygens (super-exchange) which shows up as a superhyperfine structure in the ESR (Fig. 4). In the apatites (Ms(P04)oX,M = Ca, Ba, Sr; X = C1, F) and in the LisXOq (X = P, V, As) system, the d,z ground state, the large anisotropy observed in the principal g and A tensors, and the orientation of 2 in the CrOg" tetrahedra are in agreement with the low symmetry (C,) of the host tetrahedra. In the ESR study of the compounds Cas(P04)3C1 (81, Cas(P04)aF (91,Srs(PO4)sCl(lo), and Bas(PO4)sCl (20),Cr5+ substituted for P was used as a magnetic nrohe of the local distortion:, productvi hy suhatitutional ions (Sr"', Ra", F-, C'I-r in the hvdrc~x~anatite. Ca:.rl'O+,OH structure. The -. major inorganic component of bone and teeth is basically "impure" Cas(P0g)aOH and there is substantial evidence that substitutional ions effect the physical properties of "biological apatite" through the local atomic changes they introduce in the crystal. The ESR studies show that Ca6(PO4):$, Srs(PO4)oCland Bas(POg)zC1have the hexagonal (P6sIrn) apatite structure down to 4.2' K while Cas(P04)3C1is monoclinic P 2 d 6 at liquid helium temperature. X-ray structural studies of various apatites have shown (29) that the PO4 tetrahedra have a nearly trigonal symmetry (Cia,), with a small additional distortion which leaves a mirror plane as the only element of symmetry (C,). The similarity of the resonance parameters in Tahle 1and in particular the correspondence g, < g,, A,
