Investigations On The Growth And The Physical Properties Of CaB₆and SrB₆Compounds

Alkaline-earth metal hexaborides are a class of important materials. They are wellknown for high melting temperature, high hardness, and low coefficient of thermalconductivity. These excellent properties make them widely used in various fields.In our work, we adopt solid-state reaction method to prepare the CaB₆and SrB₆polycrystals. Resistivity measurements showed that the CaB₆polycrystals obtained have atypical semi-metal electron conductive behavior. The Hall-coefficient measurementsshowed that the majority carriers in the obtained CaB₆polycrystals are electrons withcarrier density of6.82×10¹⁸electron cm-3at room temperature. The Heat Capacity of CaB₆polycrystals is well described by Debye and Einstein combined Model.High quality CaB₆and SrB₆single crystals were successfully synthesized under highpressure and temperature. They possesses cubic structure with a=4.159and4.1975,which has been characterized by XRD. The morphology of single crystals was observedby FESEM. Three kinds of shapes were observed: rods, plate-like rectangular blocks, andcubes. Based on the results of the studies, we analyse the mechanism of crystal growth.Resistivity measurements show that the CaB₆and SrB₆single crystals have a typical semi-metal electron conductive behavior. The first principles calculations confirm theexperimentally observed conducting behavior of the CaB₆and SrB₆single crystals. TheHall measurements showed that the majority carriers in the obtained CaB₆and SrB₆singlecrystals are electrons. The carrier densities are6.32×10¹⁸electron cm-3and7×10¹⁸electron cm-3at room temperature, respectively. The magnetization measurement showsthat CaB₆is paramagnetic and SrB₆is diamagnetic. The Heat Capacities of CaB₆and SrB₆are well described by Debye and Einstein combined Model.Mn-doped CaB₆single crystals were successfully obtained by means of high pressureand temperature technique. The crystal structure has been characterized by XRD.Compared with the undoped sample, the structure parameters of Mn-doped CaB₆singlecrystals decrease with the increase of the doping concentration. Magnetoresistivitymeasurements show that the samples transform from semi-metal into semiconductor withthe increase of the doping concentration. We measured the sample Hall coefficient using a four-wire method. The results show that the Mn-doped CaB₆single crystals are N-typesemiconductor. The magnetization measurement shows that the Mn-doped CaB₆sampleshave a weak ferromagnetic. The coercivity and saturation magnetic moment increase withthe increases of the doping concentration. The Heat Capacity of Mn-doped CaB₆is welldescribed by Debye and Einstein combined Model. The coefficient of electronic heatcapacity for normal state increases gradually with increases of the doping concentration,and the Debye temperature decreases gradually. The Einstein temperature is nearlyunchanged with the change of the doping concentrationMg-doped CaB₆single crystals were successfully prepared under high pressure andtemperature. The crystal structure parameters decrease with the increase of the Mg dopingconcentration. Resistivity measurements show that the Mg-doped samples aresemiconductor in the2²7K temperature range and have typical semi-metal electronconductive behavior in the27³00K temperature range. The resistivity increases graduallywith the increases of the Mg doping concentration. The Hall coefficient measurementsshow that the majority carriers in the Ca0.93Mg0.07B₆single crystals obtained is electrons.By means of Debye and Einstein combined Model, we well described the Heat Capacitiesof Mg-doped CaB₆. The results show that the coefficient of electronic heat capacity andthe Debye temperature increases gradually with increases of the Mg doping concentration,and the Einstein temperature are nearly unchanged.