High Pressure Syntheses And Characterizations Of The Rare-earth Transition Metal Oxides With The Pyrochlore Structure

By using a Kawai-type two-stage multianvil high pressure apparatus,we synthesized a series of geometrically frustrated pyrochlore oxides,including the Pt-based rare-earth pyrochlores R₂Pt₂O₇?R=Gd,Tb,Dy,Ho,Er,Yb?,Yb-based pyrochlores Yb₂B₂O₇(B=Ti_1-xSn_x,Ti_1-xGe_x,0?x?1)and Mn-based pyrochlores R₂Mn₂O₇?R=Er,Yb?.Then,we characterized their crystal and magnetic structures,and physical properties via measurements of X-ray and neutron powder diffraction,dc and ac magnetic susceptibility,as well as specific heat down to 50 mK.The main results include:?1?The classical Heisenberg pyrochlore antiferromagnets Gd₂Ge₂O₇ and Gd₂Pt₂O₇ enter a long-range antiferromagnetically ordered state through a single,strongly first-order transition at T_N=1.4 K and 1.56 K,respectively,with the specific-heat anomaly around T_N similar to that of Gd₂Sn₂O₇ rather than Gd₂Ti₂O₇.Interestingly,the low-temperature magnetic specific heat of both Gd₂Ge₂O₇ and Gd₂Pt₂O₇ was found to follow nicely the T³-dependence as seen in a conventional three-dimensional antiferromagnet with gapless spin-wave excitations.In comparison with the relatively low T_N?1 K for Gd₂Sn₂O₇ and Gd₂Ti₂O₇,we have rationalized the enhancement of T_N in terms of the reduced Gd-Gd distances for the chemically pressurized Gd₂Ge₂O₇ and the addition of extra Gd-O-Pt-O-Gd superexchange pathways through the empty Pt-e_g orbitals for Gd₂Pt₂O₇,respectively.?2?The XY-pyrochlore antiferromagnets Er₂Pt₂O₇and Yb₂Pt₂O₇ were found to undergo a long-range magnetic transition at T_N,C?0.3 K,which are ascribed to an antiferromagnetic-and ferromagnetic-type order for Er₂Pt₂O₇and Yb₂Pt₂O₇,respectively.By systematic comparisons with their respective analogue compounds Er₂B₂O₇ and Yb₂B₂O₇?B=Sn,Ti,Ge?,the observed T_N of Er₂Pt₂O₇ is much lower than that expected from the relationship of T_N versus the ionic radius of B⁴⁺for the series of Er₂B₂O₇,while the T_C of Yb₂Pt₂O₇ is highest among the series of ferromagnetic Yb₂B₂O₇?B=Sn,Pt,Ti?.Given the monotonic variation of the lattice constant as a function of B-cation size across these two series of R₂B₂O₇?R=Er,Yb?,the observed anomalous values of T_N,C in the Pt-based XY pyrochlores imply another important factor beyond the nearest-neighbor R-R distance is playing a role.In light of the anisotropic exchange interactions J_ex=<J_zz,J?,J??,J_z?>for the S-1/2 XY pyrochlores,we have rationalized these observations by considering a weakened?enhanced?antiferromagnetic planar J?(ferromagnetic Ising-like J_zz)due to strong Pt-5d and O-2p hybridization within the plane perpendicular to the local<111>direction.?3?The Ising-type pyrochlores Dy₂Pt₂O₇ and Ho₂Pt₂O₇ do not undergo any long-range magnetic order,but display characteristics of classical spin ice materials such as?Dy,Ho?₂Ti₂O₇.Interestingly,we observed the absence of nuclear Schottky anomaly and an extra peak on top of the typical hump of Ho-pyrochlore spin ices in the low-temperature specific heat of Ho₂Pt₂O₇,resulting in a low-temperature magnetic entropy exceeding the expected Rln2 for a ground doublet,but approaching to the value of^Rln3 after taking into account of a Pauling's residual entropy.Here,R stands for the ideal gas constant.Since the entropy associated with the extra peak is close to R?ln3-ln2?,we have attributed this peak to a transition from doublet to‘pseudo-triplet'state,which consists of a doublet and a singlet.We argued that it is the spatially more extended Pt-5d orbitals that modify the crystalline electric field of the non-Kramers Ho³⁺ion so as to leave the excited singlet in energy very close to the ground doublet responsible for the spin-ice behaviors.Our work thus makes Ho₂Pt₂O₇ the first promising candidate material for the Potts spin-ice model.We found that Tb₂Pt₂O₇ does not show long-range magnetic order down to 30 mK,making it another promising candidate for spin liquid as Tb₂Ti₂O₇.?4?Theoretical studies suggested that Yb-based pyrochlores Yb₂B₂O₇ may host the quantum spin ice state.However,previous experimental investigations have established that both Yb₂Sn₂O₇ and Yb₂Ti₂O₇pyrochlores form a splayed ferromagnetic?SP-FM?order below T_C=0.15 K and 0.25K,respectively,while the chemically pressurized Yb₂Ge₂O₇(the ionic radius of Ge⁴⁺:0.53?is much reduced compared to that of Ti⁴⁺:0.605?and Sn⁴⁺:0.69?)undergoes a second-order antiferromagnetic?AFM?transition at T_N=0.62 K.For the series of Yb₂(Ti_1-xGe_x)₂O₇ with an SP-FM and an AFM end members,we observed a smooth variation of the magnetic transition temperature without the expected magnetic quantum critical point.In contrast,a non-monotonic evoulution of the magnetic transition temperature with a local maximum around y=0.5 was evidenced in the series of Yb₂(Ti_1-ySn_y)₂O₇ with two end members having the similar SP-FM ground state.A systematic study on the magneic field effects shows that the magnetic transition temperatures first decrease and then increase upon increasing the external magnetic field for the intermediate compositions of both Yb₂(Ti_1-xGe_x)₂O₇ and Yb₂(Ti_1-ySn_y)₂O₇,which indicate that the static lattice disorder plays an important role in tuning the magnetic ground state.?5?We discovered a large and reversible magnetocaloric effect?MCE?in the cubic pyrochlore Er₂Mn₂O₇ near its second-order ferromagnetic?FM?transition at T_c?34 K;under the magnetic field change of 1 and 5 T,the maximum magnetic entropy change-?35?S_M is 5.27 and 16.1 J·Kg^-1·K^-1,and the estimated magnetic refrigerant capacity reaches 68 and 522 J·kg^-1,respectively.These latter values are among the largest for the known MCE materials.The observed giant and reversible MCE in Er₂Mn₂O₇ is mainly attributed to the large saturation moment of 18.9?_B/f.u.owing to a simultaneous FM ordering of the rear-earth Er³⁺and transition-metal Mn⁴⁺localized moments,unlike most MCE materials involving large-moment magnetic species on one sublattice.Our results demonstrate that the cubic R₂Mn₂O₇ pyrochlore is a promising material system for developing high-performance MCE materials that can exhibit a strongly coupled FM transition involving two magnetic sublattices of large local moments in a single-phase material.