Hydrothermal synthesis of ThO2, UxTh1-xO2, and UOx at temperatures between 670°C and 700°C has been demonstrated. Synthesis at these temperatures is 50-80°C below prior growth studies and represents a new lower bound of successful growth. ThO2 single crystals of dimensions 6.49mm x 4.89mm x 3.89 mm and weighing 0.633g have been synthesized at average growth rates near 0.125mm/week. Single crystal UxTh1-xO2 crystals with mole fractions up to x≈0.30 have also been grown. The largest alloyed crystal with mole fraction x≈0.23 has dimensions of 2.97mm x 3.23mm x ∼3mm and recorded average growth rates near 0.2mm/week. Four structures were solved from X-ray diffraction data and their crystallographic data reported here. Rocking curve analysis determined a dislocation density of 1.2×109 cm-2.

The electronic properties of ThO2 single crystals were studied using x-ray photoemission spectroscopy (XPS). The XPS results show that the Th 4f core level is in an oxidation state that is consistent with that expected for Th in ThO2. The effective Debye temperature is estimated from the temperature dependent photoemission intensities of the Th 4f core level over the temperature range of 290 to 360 K. A Debye temperature of 468±32 K has been determined.

The relationship between the optical properties and microstructure of GaN is of great interest due to the important optical and electronic applications of this material. Several different studies have been reported attempting to link the low temperature photoluminescence (PL) peak at ∼3.4 eV to the presence of various microstructural defects. However, no clear systematic studies have been reported establishing such a link for the PL peak observed at ∼3.2 eV. In this paper, we present evidence linking the ∼3.4 eV PL peak to the presence of a thin layer of cubic phase associated with basal plane stacking faults (BSF). This relationship is mainly established by studying a series of ammonothermally-grown GaN bulk crystals. The existence and strength of the ∼3.4 eV peak are found to be related to the I2 type BSF (RI2=1/3<1>) observed in these samples. To investigate the relationship between the ∼3.2 eV peak and structural defects, a series of GaN epilayers grown on either SiC or sapphire (of various off-cut angles) was investigated by TEM and PL spectroscopy. Samples grown on 3.5° off-cut SiC and 5° and 9° off-cut sapphire substrates exhibit PL peaks near ∼3.2 and ∼3.4 eV, which are absent in the on-axis SiC and sapphire cases. TEM shows that the former group of samples has defect configurations consisting of prismatic stacking faults (PSFs) folding back and forth between two different {1120} planes connected by stair rod dislocations, which in turn fold onto to I1 type BSFs again with stair rod dislocations at the fault intersections. The ∼3.2 eV PL peaks are proposed to possibly arise from transitions involving the PSFs and the stair rods associated with their mutual intersections and their intersections with the BSFs. The ∼3.4 eV peak is again attributed to the thin layer of cubic phase associated with the I1 type BSF (three bilayers as opposed to four bilayers for the I2 type BSF).

Growth of GaN bulk crystals under ammonothermal conditions has been developed. The experiments were performed in ammono-basic solutions in high nickel content autoclaves for up to 3 weeks. Nutrients were crystalline GaN made from vapor phase transport growth. Single crystal clusters of GaN on the order of 500 μm – 1 mm long were obtained. These crystals were spontaneously nucleated on the walls of the autoclave. Transport growth on polycrystalline GaN seeds and single crystal HVPE seeds was also achieved. GaN has a high solubility in ammono-basic solutions, on the order of several weight percent. The ammonothermal crystals were characterized by photoluminescence (PL), X-ray diffraction and SEM.

The transition metal pentatellurides HfTe5 and ZrTe5 exhibit a broad resistive anomaly as a function of temperature. This behavior is also reflected in the thermopower as it changes from a large positive value below room temperature to a large negative value at lower temperatures with the zero crossing corresponding well with the peak temperature of the resistive anomaly. The large values of the thermopower at low temperatures (T ≈ 150 K) have made these materials attractive for investigation for potential low temperature thermoelectric applications. The magnitude of the resistive peak and the peak temperature are highly sensitive to doping as well as external influences such as magnetic field and pressure. In this study we examine the effect of doping with various rare earth elements (RE = Ce, Sm and Dy) and the subsequent effects on the electrical resistivity and the thermopower. These results will be discussed in relation to the thermoelectric performance of these materials.

The thermoelectric figure of merit, ZT = α2σT/λ, has been measured for pentatelluride single crystals of HfTe5, ZrTe5, as well as Se substituted pentatellurides. The parent materials, HfTe5 and ZrTe5, exhibit relatively large p- and n- type thermopower, |a| > 125 μV/K, and low resistivity, ρ ≤ 1 mΩ•cm. These values lead to a large power factor (α2σT) which is substantially increased with proper Se substitution on the Te sites. The thermal conductivity of these needle-like crystals has also been measured as a function of temperature from 10 K ≤ T ≤ 300 K. The room temperature figure of merit for these materials varies from ZT “0.1 for the parent materials to ZT ≈ 0.25 for Se substituted samples. These results as well as experimental procedures will be presented and discussed.

The pentatelluride materials (Hf1-XZrXTe5) have recently garnered much interest as a potential low temperature thermoelectric material. Their power factor exceeds that of the current Bi2Te3 materials over the temperature range 150 K < T < 350 K. A formidable challenge has been the capability of measuring the thermal conductivity of small needle-like samples (2.0 × 0.05 × 0.1 mm3) such as pentatellurides (HfXZr1-XTe5) due to heat loss and radiation effects. However in order to fully evaluate any material for potential thermoelectric use, the determination of the thermal conductivity of the material is necessary. We have recently developed a new technique called the parallel thermal conductance (PTC) technique to measure the thermal conductivity of such small samples. In this paper we describe the PTC method and measurements of the thermal conductivity of the pentatelluride materials will be presented for the first time. The potential of these materials for low temperature thermoelectric applications will be further evaluated given these results as well as future work and directions will be discussed.

The thermoelectric properties (resistivity and thermopower) of single crystals of the low-dimensional pentatelluride materials, Hffe5 and ZrTe5, have been measured as a function of temperature from 10K < T < 320K. Both parent materials exhibit a resistive transition peak, Tp ≈ 80K for HfTe5 and Tp ≈ 145K for ZrTe5. Each display a large positive (p-type) thermopower (α ≥ +125μV/K) around room temperature, which undergoes a change to a large negative (n-type) thermopower (α≤-125μV/K) below the peak temperature. The magnitude of this resistive anomaly is typically 3–7 times the room temperature value of ≈ 1 mΩ•cm. Through isoelectronic substitution of Zr for Hf (Hf1-xZrxTe5), a systematic shift is observed in Tp as the Zr concentration increases. Small Ti substitution for Hf and Zr affects the electronic properties strongly, producing a substantial reduction in Tp for either parent compound. However, the large values of thermopower and the magnitude of the resistive peak remain essentially unchanged. Substitutions of Se or Sb on the Te sites greatly affects the electronic behavior of the parent materials. Se doping increases the thermopower values by ≈20% while decreasing the resistivity by as much as 25%. These effects double the power factor, α2σT, of the parent materials. Small Sb substitutions appear to completely suppress the resistive anomaly. These features in the resistivity and thermopower signal a large degree of tunability in the temperature range of operation. The potential of these materials as candidates for low temperature thermoelectric applications will be discussed.

A new procedure for the synthesis of transition metal sulfosalts in hydrothermal brines is presented. Large, well formed crystals of metal antimony sulfides can be prepared under appropriate conditions in supercritical brines. A new compound prepared by this method, MnSb2S4, is structurally characterized.