Abstract
This paper presents the physical properties of a nonstandard orthorhombic form of , including a comparison with the isostructural orthorhombic niobate , and with the usual monoclinic polymorph. Orthorhombic () is obtained under extreme conditions of high pressure (6.7 GPa) and high temperature (). A negative Curie-Weiss temperature is observed, implying dominant antiferromagnetic interactions at high temperatures, in contrast to the positive of the monoclinic form. Specific-heat measurements are reported down to 1.8 K for all three compounds, and corroborate the magnetic-transition temperatures obtained from susceptibility data. Orthorhombic presents a transition to an ordered antiferromagnetic state at K. Its magnetic structure, determined by neutron diffraction, is unique among the columbite compounds, being characterized by a commensurate propagation vector . It presents antiferromagnetic chains running along the axis, but with a different spin pattern in comparison to the chains observed in . By a comparative discussion of our observations in this three compounds, we are able to highlight the interplay between competing interactions and dimensionality that yield their magnetic properties.
- Received 23 September 2016
DOI:https://doi.org/10.1103/PhysRevB.95.024419
©2017 American Physical Society