The thermodynamic temperature of the point of inflection of the melting transition of Re-C, Pt-C and Co-C eutectics has been determined to be 2747.84 ± 0.35 K, 2011.43 ± 0.18 K and 1597.39 ± 0.13 K, respectively, and the thermodynamic temperature of the freezing transition of Cu has been determined to be 1357.80 ± 0.08 K, where the ± symbol represents 95% coverage. These results are the best consensus estimates obtained from measurements made using various spectroradiometric primary thermometry techniques by nine different national metrology institutes. The good agreement between the institutes suggests that spectroradiometric thermometry techniques are sufficiently mature (at least in those institutes) to allow the direct realization of thermodynamic temperature above 1234 K (rather than the use of a temperature scale) and that metal-carbon eutectics can be used as high-temperature fixed points for thermodynamic temperature dissemination. The results directly support the developing mise en pratique for the definition of the kelvin to include direct measurement of thermodynamic temperature.

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The growth of rare-earth (RE ) Nd, Tb, Ho, Er, Tm, and Yb) doped KTiOPO 4 (KTP) single crystals from high-temperature solutions [K 2 O-P 2 O 5 -TiO 2 -RE 2 O 3 (or Tb 4 O 7 )] was investigated. The concentration regions of crystallization of RE-doped KTP, the maximum (critical) RE concentrations in the solution without losing the KTP phase and the corresponding upper limit of RE ions incorporated into the KTP lattice were obtained experimentally. The crystallization regions of RE-doped KTP are significantly narrower than the region of undoped KTP. Above a certain critical RE concentration in the solution, RE phosphate phases crystallize. The highest concentration of RE in the crystal is in the range 50-600 ppm and strongly depends on the RE ionic radius. Some double substitutions with Nd and a codopant element can significantly change this concentration. In addition, RE-doped KTP single crystals were grown by the top seeded solution growth (TSSG) technique, and their experimental conditions were compared with those of undoped KTP single-crystal growth. Optical absorption spectra for Ho-, Er-, and Tm-doped KTP were obtained and discussed.