A₂MnXO₄ Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties

Abstract

Four new manganese germanates and silicates, AMnGeO (A = Li, Na) and AMnSiO (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. LiMnGeO is orthorhombic (Pmn2) layered, isostructural with LiCdGeO, and the three other compounds are monoclinic (Pn) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (Pn AMXO, Pna2 and Pbca AMO), average bond angles on bridging oxygens (here, Mn-O-X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (Fd3̅m) structure typified by CsAlO. The sublattices of the magnetic Mn ions in both structure types under study (Pmn2 and Pn) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus LiMnSiO were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and electron spin resonance measurements and also by performing energy-mapping analysis to evaluate their spin exchange constants. AgMnSiO remains paramagnetic down to 2 K, but AMnXO (A = Li, Na; X = Si, Ge) undergo a three-dimensional antiferromagnetic ordering. All five phases exhibit short-range AFM ordering correlations, hence showing them to be low-dimensional magnets and a magnetic field induced spin-reorientation transition at T < T for all AFM phases. We constructed the magnetic phase diagrams for AMnXO (A = Li, Na; X = Si, Ge) on the basis of the thermodynamic data in magnetic fields up to 9 T. The magnetic properties of all five phases experimentally determined are well explained by their spin exchange constants evaluated by performing energy-mapping analysis.

Defects, Dopants and Sodium Mobility in Na2MnSiO4

“…16 In the Fig. 1, crystal structure of Na 2 MnSiO 4 is shown together with bonding nature of all cations forming corner-sharing tetrahedral units with four O atoms.…”

Long-range magnetic ordering in Li2MnGeO4 and precursor short-range spin correlations

“…Polycrystalline Li 2 MnGeO 4 was prepared by conventional solid-state synthesis, as previously reported [14]. For the present neutron and synchrotron diffraction study, a larger batch was prepared from the same reagents by the same route.…”

“…1. The data were successfully analyzed using the initial structure model previously determined using laboratory x-ray diffraction [14]. The refined atomic coordinates with thermal parameters in view of the Pmn2 1 space group are indicated in Table I.…”

“…Recently, a long-range antiferromagnetic (AFM) ordering was reported only experimentally by means of neutron powder diffraction for β I -Li 2 CoSiO 4 and γ 0 -Li 2 MnSiO 4 [13]. Besides, bulk magnetic properties and DFT calculations of spin exchange constants were published for Li 2 MnSiO 4 , Na 2 MnSiO 4 , Li 2 MnGeO 4 , and Na 2 MnGeO 4 [14]. In particular, it was found that Li 2 MnGeO 4 undergoes a three-dimensional antiferromagnetic ordering at the Néel temperature T N = 8.1 ± 0.5 K with Weiss temperature = −27 K. The negative value indicates predominantly mainly AFM interactions in the compound.…”

“…In particular, high ionic conductivity is caused by the presence of the allocated alkalimetal ion layers. The ordering of oxygen tetrahedra with M/X ions in the center leads to the spin-frustrated triangles of manganese ions [14]. With antiferromagnetic interactions, there is a spin coupling conflict between the nearest neighbors, and such a system becomes magnetically frustrated.…”