Tatsuo Takahashi

Growth of FeS single crystals from nonstoichiometric melts by normal freezing

Stoichiometric FeS single crystals were grown from iron-rich nonstoichiometric melts by normal freezing. These crystals were grown to investigate the alpha transformation and associated properties of this material. All experiments were performed in an rf-furnace filled with 700 Torr Ar gas using a pointed-bottom graphite crucible. Using melt compositions of 51.5 at % and 52.0 at % Fe, good crystals (max. size: 10 mm in dia. 20 mm in length) were obtained at a cooling rate of 15–20 °C/hr. With increasing iron concentration, stable growth was difficult to maintain at the same cooling rate and the FeS single phase zone was limited to a height of only 3–5 mm from the bottom of the crucible. Chemical analyses of these crystals indicated that they are indeed stoichiometric (FeS1.00–FeS1.01). Both differential thermal analysis and electrical measurements indicated that their characteristics are considerably different from the single crystals prepared by recrystallization.

Growth of HgCr2Se4 single crystals by chemical transport

Abstract Single crystals of undoped, Ag-doped and In-doped HgCr 2 Se 4 have been grown by chemical transport from the HgCr 2 Se 4 - CrCl 3 system. A mixture of HgCr 2 Se 4 , either undoped or doped, and CrCl 3 was sealed in an evacuated quartz ampoule and heated in a two-zone furnace ( T high ~ 700 °C, and T low ~ 670 °C). Single crystals of up to 3 mm on an edge, with a truncated octahedral shape, were found on the wall of the low temperature end of the ampoule. The room temperature Seebeck coefficients of these single crystals have been measured and compared with those obtained for pressure-sintered samples.

Growth of cadmium boracite single crystals by chemical vapor transport

Abstract A new and simple method to grow Cd 3 B 7 O 13 X (X=Cl or Br) single crystals by chemical vapor transport was developed. In an evacuated quartz ampoule, CdO and CdX 2 were placed at one end and B 2 O 3 was placed at the other end. The ampoule was heated in a two-zone furnace, keeping the Cd compound end at the higher temperature ( T H ) and the B 2 O 3 end at the lower temperature ( T L ). Singles crystals of polyhedral shape (max. edge length ~5 mm) were found to grow at the middle section of the ampoule. The optimum temperatures were T H =850°C, T L =800°C for Cd-Cl boracite and T H =820°C, T L = 770°C for Cd-Br boracite, respectively. The crystals were found to have twin lamellar structure. The lamellae were parallel to the {110} planes of the high temperature cubic phase. DTA measurements showed an anomaly at a temperature below the transition temperature to the cubic structure. Such an anomaly seems to suggest the occurence of a higher order phase transition at the temperature.

Synthesis and some properties of RCr3S6 crystals ( = Y, Gd, Dy, Ho and Er)

Abstract A series of new compounds, RCr 3 S 6 (R = Y, Gd, Dy, Ho and Er), has been synthesized from the high temperature reaction of RCrS 3 and CrCl 3 or I 2 . X-ray measurements showed that these crystals have an isostructural orthorhombic structure. Magnetic measurements in the temperature range 78–500 δK indicated that all RCr 3 S 6 undergo a magnetic transition at a temperature between 130 and 150 δK.

Switching in cadmium boracite single crystals

We have found that a highly insulative Cd boracite crystal becomes conductive when a certain electric voltage is applied at a high temperature (≳300 °C). The threshold voltage for switching is smaller than a few volts for an electrode distance of a few tenths of a millimeter. This corresponds to an electric field of 102–103 V/cm and is much smaller than the dielectric breakdown voltage for an insulator such as boracite. The insulative state is restored after the removal of voltage. The switching has also been investigated by a pulse technique. In this case, two different types of switching are observed. Unstable switching is observed when the pulse voltage is slightly above the threshold and is characterized by a delay time before switching and a residual current after the removal of a voltage pulse; whereas stable switching takes place when the voltage becomes sufficiently high.

Cathodoluminescence of Terbium Activated Metaphosphate Glasses

Cathodoluminescent properties of several alkali and alkaline earth terbium metaphosphate glasses were investigated at an applied voltage of 100 V to 2500 V in a vacuum diode structure. Light outputs were compared with that from a Zn2SiO4:Mn (Pl) phosphor.