Lanthanum and cerium bromides and chlorides form isomorphous alloy systems with the UCl3
type structure. These scintillating alloys exhibit high luminosity and proportional response, making
them the first scintillators comparable to room temperature semiconductors for gamma spectroscopy;
Ce(III) activated lanthanum bromide has recently enabled scintillating gamma ray spectrometers
with < 3% FWHM energy resolutions at 662 keV. However brittle fracture of these materials
impedes development of large volume crystals. Low fracture stress and perfect cleavage along
prismatic planes cause material cracking during and after crystal growth. These and other properties
pose challenges for material production and post processing; therefore, understanding mechanical
behavior is key to fabricating large single crystals, and engineering of robust detectors and systems.
Recent progress on basic structure and properties of the lanthanide halides is reported here,
including thermomechanical and thermogravimetric analyses, hygroscopicity, yield strength, and
fracture toughness. Observations including reversible hydrate formation under atmospheric pressure,
loss of stoichiometry at high temperature, anisotropic thermal expansion, reactivity towards common
crucible materials, and crack initiation and propagation under applied loads are reported. The
fundamental physical and chemical properties of this system introduce challenges for material
processing, scale-up, and detector fabrication.
Analysis of the symmetry and crystal structure of this system suggests possible mechanisms for
deformation and crack initiation under stress. The low c/a ratio and low symmetry relative to
traditional scintillators indicate limited and highly anisotropic plasticity cause redistribution of
residual process stress to cleavage planes, initiating fracture. This proposed failure mechanism and
its implications for scale up to large diameter crystal growth are also discussed.