We use neutron powder diffraction to investigate the magnetic and crystalline structure of Cr_(2)GaN.A magnetic phase transition is identified at T≈170 K,whereas no trace of structural transition is observed down to ...We use neutron powder diffraction to investigate the magnetic and crystalline structure of Cr_(2)GaN.A magnetic phase transition is identified at T≈170 K,whereas no trace of structural transition is observed down to 6 K.Combining Rietveld refinement with irreducible representations,the spin configuration of Cr ions in Cr_(2)GaN is depicted as an incommensurate sinusoidal modulated structure characterized by a propagating vector k=(0.365,0.365,0).Upon warming up to the paramagnetic state,the magnetic order parameter closely resembles to the temperature dependence of c-axis lattice parameter,suggesting strong magneto-elastic coupling in this compound.Therefore,Cr_(2)Ga N provides a potential platform for exploration of magnetically tuned properties such as magnetoelectric,magnetostrictive and magnetocaloric effects,as well as their applications.展开更多
Neutron diffraction studies of the low-temperature relaxor ferroelectric phases of[NH_(4)^(+)]M(HCO_(2))_(3),where M=Mn^(2+)and Zn^(2+),show that a third of the NH_(4)^(+)t cations remain subtly structurally disordere...Neutron diffraction studies of the low-temperature relaxor ferroelectric phases of[NH_(4)^(+)]M(HCO_(2))_(3),where M=Mn^(2+)and Zn^(2+),show that a third of the NH_(4)^(+)t cations remain subtly structurally disordered to low temperature.All NH_(4)^(+) cations within the channels are well separated from each other,with significant hydrogen bonds only with the anionic M(HCO_(2))_(3) framework.Complementary studies of the dynamics using ^(2)H solid state NMR and quasielastic neutron scattering indicate significant rotational motion in both paraelectric and ferroelectric phases,which evolves gradually with increasing temperature with no abrupt change at the phase transition.Nudged elastic band calculations suggest that the activation barrier for flipping between"up"and"down"orientations of the NH_(4)^(+) cations is low in the ferroelectric phase,with the NH_(4)^(+) cations primarily interacting with the framework rather than neighbouring NH_(4)^(+)*cations.It is likely this motion that is responsible for scrambling the NH_(4)^(+) cation orientation locally in the ferroelectric phase.We propose that this disorder,with the same basic motion active above and below the phase transition,induces the significant dielectric relaxation in these materials.This suggests that orientational disorder may be an effective substitution for compositional disorder commonly associated with relaxor ferroelectrics in molecular materials.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.11822411,12061130200,11961160699,11974392,and 52101236)the National Key Research and Development Program of China(Grant Nos.2018YFA0704200,2017YFA0303100,and 2020YFA0406003)+4 种基金the Strategic Priority Research Program(B)of the CAS(GrantNo.XDB25000000)K.C.Wong Education Foundation(Grant No.GJTD-2020-01)the Youth Innovation Promotion Association of the CAS(Grant No.Y202001)Beijing Natural Science Foundation(Grant No.JQ19002)the Newton Advanced Fellowship funding from the Royal Society of UK(Grant No.NAFR1201248)。
文摘We use neutron powder diffraction to investigate the magnetic and crystalline structure of Cr_(2)GaN.A magnetic phase transition is identified at T≈170 K,whereas no trace of structural transition is observed down to 6 K.Combining Rietveld refinement with irreducible representations,the spin configuration of Cr ions in Cr_(2)GaN is depicted as an incommensurate sinusoidal modulated structure characterized by a propagating vector k=(0.365,0.365,0).Upon warming up to the paramagnetic state,the magnetic order parameter closely resembles to the temperature dependence of c-axis lattice parameter,suggesting strong magneto-elastic coupling in this compound.Therefore,Cr_(2)Ga N provides a potential platform for exploration of magnetically tuned properties such as magnetoelectric,magnetostrictive and magnetocaloric effects,as well as their applications.
基金supported during this work through a Project Grant from the Leverhulme Trust(RPG-2018-288)Neutron diffraction and QENS experiments at the ISiS Pulsed Neutron and Muon Source were supported by a beamtime allocation from the Science and Technology Facilities Council(RB 1820062 and 1910402)We are grateful to the UK Materials and Molecular Modelling Hub for computational resources,which is partially fundedby EPSRC(EP/T022213/1,EP/W032260/1 and EP/P020194/1).
文摘Neutron diffraction studies of the low-temperature relaxor ferroelectric phases of[NH_(4)^(+)]M(HCO_(2))_(3),where M=Mn^(2+)and Zn^(2+),show that a third of the NH_(4)^(+)t cations remain subtly structurally disordered to low temperature.All NH_(4)^(+) cations within the channels are well separated from each other,with significant hydrogen bonds only with the anionic M(HCO_(2))_(3) framework.Complementary studies of the dynamics using ^(2)H solid state NMR and quasielastic neutron scattering indicate significant rotational motion in both paraelectric and ferroelectric phases,which evolves gradually with increasing temperature with no abrupt change at the phase transition.Nudged elastic band calculations suggest that the activation barrier for flipping between"up"and"down"orientations of the NH_(4)^(+) cations is low in the ferroelectric phase,with the NH_(4)^(+) cations primarily interacting with the framework rather than neighbouring NH_(4)^(+)*cations.It is likely this motion that is responsible for scrambling the NH_(4)^(+) cation orientation locally in the ferroelectric phase.We propose that this disorder,with the same basic motion active above and below the phase transition,induces the significant dielectric relaxation in these materials.This suggests that orientational disorder may be an effective substitution for compositional disorder commonly associated with relaxor ferroelectrics in molecular materials.
