Crystal structure evolution of BaBrCl and BaBrCl:5%Eu up to 1073 K by neutron diffraction

Abstract

BaBrCl:Eu is a promising scintillator material; however, the crystal growth yield must be improved for it to become commercially viable. This study measures strain accumulations in the crystal lattice which can contribute to cracking during post-growth cooling. Neutron diffraction is used to measure the crystal structure of undoped and 5 mol% europium-doped BaBrCl from 303 to 1073 K, approaching the melting point. Rietveld analysis of these data provides the temperature dependence of the thermal and chemical strain in BaBrCl. In particular, anisotropic thermal expansion is measured, with expansion along thebaxis nearly double the expansion along theaandcaxes. Additionally, the chemical strain from the incorporation of europium atoms peaks around 673 K, explaining cracking frequently observed in that temperature range.

The crystal structure and temperature dependence of the elpasolite Tl2LiYCl6

“…Understanding the crystal structure and how it evolves from the melting point down to room temperature is useful for confronting challenges during bulk crystal growth. Single crystals may crack during post-growth cooling, due to phase transitions, anisotropic thermal expansion or lattice strain from the introduction of dopants (Doty et al, 2007;Onken et al, 2018). For elpasolites in particular, structural transitions involving the tilting or distortion of the LiX 6 and MX 6 octahedra have been observed (Flerov et al, 1998), and some of these transitions have been associated with twinning ISSN 1600-5767 (Bezbtosikov et al, 1983).…”

Analysis of chemical stress and the propensity for cracking during the vertical Bridgman growth of BaBrCl:Eu

“…where σ ij are components of the stress tensor, ij are components of the strain tensor, α is the chemical expansion coefficient, and C(r, z) is the europium dopant concentration distribution that arises from segregation during growth. We note that Onken et al [42] reported anisotropic Eu doping strain along different BaBrCl lattice directions; however, we have employed an averaged, isotropic chemical expansion coefficient, α, to simply our analysis. The coefficients E ij correspond to the fundamental elastic constants of an orthorhombic crystal whose c-axis is aligned with the z-axis of a Cartesian coordinate system [11], with indices {1, 2, 3} corresponding to {x, y, z}.…”

“…Their results indicated a strong correlation between Eu distribution and cracking and that cracks originated from clusters where Eu concentration gradients were large. Later, Onken et al [42] used neutron diffraction to measure the lattice structure of undoped and 5% Eu-doped BaBrCl crystals at elevated temperatures. They reported anisotropic thermal expansion that produces thermal strain and found significant chemical strain caused by the presence of europium dopant atoms substituting at the barium site.…”

The crystal structure of TlMgCl3 from 290 K to 725 K

“…To yield reliable absolute lattice parameters, the DIFC instrument calibration parameters were fitted for the room-temperature data using the lattice parameters from SC-XRD and were kept constant for the rest of the ND data at higher temperatures. For more details on the data collection and refinement of these neutron diffraction data, see Onken et al (2018).…”