Influence of oxygen on electrical properties of CaF₂ crystals

Abstract

The 0,ions appear as an unwanted impurity that contaminates the crystals of alkaline and rare earth fluorides during their preparation. The easy incorporation into the lattice and difficulties with their detection make the measurement of electrical properties problematical. I n order to elucidate the influence of oxygen on CaF, crystals we have measured electrical conductivity. The crystals were prepared from a raw material of various moisture content, or they were doped by CaO, respectively. The measured courses exhibit equal activation energy E = 0.64 eV in the middle range. We thus conclude that synthetizing the crystal contaminated by moisture yields incorporation of 02-ions only. We proved that even a two hours delay of melted raw material in gaseous products of Teflon pyrolysis does not suffice to remove oxygen. The method is effective only when applied after drying of raw material in vacuum.02--Ionen treten als unerwunschte Beimengung auf, die Alkali-und Seltenerd-Fluorid-Kristalle wahrend ihrer Herstellung verunreinigen. Der leichte Einbau in das Gitter und die Schwierigkeiten bei ihrem Nachweis machen die Messung ihrer elektrischen Eigenschaften problematisch. Zur Aufkliirung des Einflusses des Sauerstoffs auf CaF,-Kristalle haben wir die elektrische Leitflhigkeit gemessen. Die Kristalle wurden aus Rohstoffen verschiedenen Feuchtigkeitsgehalts hergestellt, oder sie wurden mit CaO dotiert. Die gemessenen Verlaufe zeigen gleiche Aktivierungsenergie von E = 0,64 eV im mittleren Gebiet. Wir schlieaen daraus, dal3 die Herstellung von Kristallen in Anwesenheit von Feuchtigkeit allein den Einbau von 02--Ionen verursacht. Wir konnten nachweisen, da13 sogar ein zweistundiger Verbleib des geschmolzenen Rohmaterials in den gasformigen Produkten der Teflonpyrolyse nicht ausreichen, den Sauerstoff zu entfernen. Die Methode ist nur dann wirksam, wenn sie nach der Trocknung des Rohmaterials im Vakuum angewendet wird.

Electrical Conductivity of Fluorite and Fluorine Conduction

“…In general, the data trends of the conductivities determined for synthesized [24][25][26][27][28][29][30][31][32] and natural (this study) fluorite resemble each other, although the exact conductivity values vary by a factor of 2 to 10 depending on temperature (Figure 4). Furthermore, the activation enthalpies of electrical conduction in synthesized fluorite are usually~80-100 kJ/mol or~180-190 kJ/mol at low to moderate temperatures (e.g.,~400-650 • C) [24][25][26][27][28][29][30][31][32], which are remarkably different from the measured value of~130 kJ/mol for natural fluorite (this study). In particular, some previous reports for synthesized fluorite demonstrate apparent kinks of conductivity data in their Arrhenius plots between 400 and 600 • C, which is not observed by our measurements for natural fluorite (Figure 4).…”

“…The electrical property of fluorite has been widely investigated by researchers from materials science, wholly or partly due to its commercial potential as a laser material when doped with rare Earth impurities, uranium and/or other elements [24][25][26][27][28][29][30][31][32]. Typically, nearly all available studies were carried out on synthesized fluorite, in either polycrystalline or single crystal forms.…”

“…Fluorite samples in available reports were usually pure CaF 2 grown in laboratory, with minor impurities from the reagent materials [24][25][26][27][28][29][30][31][32], and our fluorite was nearly ideal in CaF 2 composition, with negligible amounts of other elements ( Table 1). The differences in electrical property between synthesized fluorite and natural fluorite ( Figure 4) may thus be accounted for by one or a few of the following possibilities: (1) different forms of fluorite, i.e., polycrystalline vs. single crystal.…”

“…It has been proposed in available reports that the electrical conduction in (synthesized) fluorite is controlled by anti-Frenkel defects involving negative vacancies and interstitials [24,29], thermal stress or oxygen introduced in the growth of fluorite [28,31]. Also, it has been argued that different conduction mechanisms are active at different temperatures, e.g., extrinsic vacancy conduction at low temperature and intrinsic vacancy conduction at relatively high temperatures, with a transition at 450-600 • C [25,28,30,32,33].…”

Charging phenomena in low-voltage electron microscopy of laser-fractured fluoride surfaces

“…Here, we apply the method of electronbeam-induced voltage contrast ͑EBIVC͒, exploiting the fact that secondary electron ͑SE͒ emission sensitively depends on the charge state of the surface. 10,11 Local variations of the charge translate into SE imaging contrasts.…”