Single crystal growth and low temperature properties of triangular lattice antiferromagnet Ca3NiNb2O9

Single crystals of the triangular lattice Heisenberg antiferromagnet (TLHAF) Ca3NiNb2O9 and its non – magnetic analogue Ca3MgNb2O9 were grown using the four - mirror optical float zone furnace. Here we have studied the influence of various growth atmospheres and pulling rates on the crystal quality and phase homogeneity. Both the compounds melt congruently and the grown crystal specimens suffer from cracks when the crystal boule is cooled, thus making it difficult to obtain large single crystalline pieces. To overcome this difficulty, an after – heater was used to reduce the formation of cracks. Interestingly, the crystal boule of Ca3NiNb2O9 exhibits two regions of distinct colors and optical opacities i.e. opaque black and translucent green. Further, the as - grown, opaque, black specimen was subjected to air annealing at 1200oC for 24hrs, wherein its appearance changed from black to translucent green, without any change of weight. Low temperature specific heat and magnetization measurements carried out on the as - grown black sample (AGO) and the air annealed sample (ANT) revealed contrasting ground state properties. While the as - grown specimen exhibits a broad hump close to 10 K in specific heat measurement, the air annealed sample showcases two transitions at TN1 = 4.6 K and TN2 = 4.3 K, in agreement with the previous reports by J Ma et al 1. Moreover, room temperature synchrotron XRD further revealed that AGO specimen crystallizes in a higher symmetric orthorhombic space group while the ANT specimen crystallizes in the reported monoclinic space group. Thus, the ground state of this triangular lattice system is very sensitive to the growth conditions and annealing experiments help recover the properties reported for the polycrystalline sample.