H.G. Gallagher

Dislocation Slip Systems in Pentaerythritol Tetranitrate (PETN) and Cyclotrimethylene Trinitramine (RDX) [and Discussion]

An exam ination has been made of the orientation dependence of microhardness determined using a Knoop indenter on the principal habit faces of RDX ({210}) and PETN ({110}). The variations found (RDX , 32-44 kg mm-2; PETN, 13-24 kg mm-2) were consistent with the crystallographic symmetry of the solids and confirm previous estimates obtained using a Vickers indenter. On the basis that the principal deformation mechanism is dislocation slip and that the variation in hardness reflects the orientation of the dominant slip systems to the surface and hence to the faces of the indenter, a theoretical assessment was made using the models of F. W. Daniels and C. G. Dunn and C. A. Brookes, J. B. O’Neill and B.A.W. Redfern of the expected variation in hardness due to this cause. Satisfactory agreement with experiment was obtained using slip systems of the types (010) [001] and {021} [100] for RDX and (110) [111]-type for PETN . This is the first time that this type of analysis has been attempted for such highly anisotropic systems. These assignments were confirmed by etching and by X-ray topographic analysis of the distribution of dislocation alignments around the hardness impressions. The difference in mechanical behaviour of these solids and its possible role in sensitivity to detonation is discussed on the basis of the relative ease of migration of these dislocation systems.

Potential laser gain media with the stoichiometric formula RETiNbO6

Abstract The laser heated pedestal growth (LHPG) technique has been used to grow miniature crystals of the mixed niobates, RETiNbO 6 with RE = Nd, Pr and Er, having typical dimensions of 0.5 mm diameter and 10 mm in length. The Nd and Pr compounds grow in the aeschynite structure, whereas the Er compound grows with the euxenite structure, The crystals grown by the LHPG technique were characterised by measurement of their X-ray diffraction patterns and optical absorption and photoluminescence spectra. The latter show very high absorption coefficients for the rare earth ions and strong luminescence signals. Brief reports are presented of these spectra, and discussed in terms of the potential of RETiNbO 6 single crystals as laser gain media.

The crystal growth and perfection of 2,4,6-trinitrotoluene

Abstract Large crystals of TNT were grown from ethyl acetate solution by both temperature lowering and solvent evaporation. The perfection of crystals grown from seeds under carefully controlled conditions was generally higher than those prepared by uncontrolled solvent evaporation. Examination by X-ray topography revealed the crystals to have a characteristic growth induced defect structure comprising growth sectors and boundaries, growth banding, solvent inclusions and dislocations. Twins and stacking faults (SF) were also observed. Many of the defects noted in the topographs can be attributed to impurities. The influence of the highly anisotropic crystal structure on the nature of growth defects is discussed. A structural model proposed to explain twinning and SF formation is partially supported by topographic evidence.

Crystal growth and optical characterisation of Cr3+-doped YAl3(BO3)4

Abstract This paper reports the high temperature top-seeded solution growth (TSSG) of large single crystals of Cr 3+ -doped YAl 3 (BO 3 ) 4 from a K 2 Mo 3 O 10 B 2 O 3 flux cooled at a rate of 2–4 K per day in the temperature range 950–1060°C, using a crystal rotation rate of 4–6 rpm. Optical absorption measurements at T = 300 K reveal broad bands associated with transitions from the 4 A 2 ground state of Cr 3+ to the 4 T 2 and 4 T 1 excited states. The doublet R lines and their one-phonon sideband are clearly resolved at room temperature in both absorption and luminescence spectra: the splitting of the R lines demonstrates the effect of the even-parity trigonal distortions of their environment on the 4 A 2 and 2 E levels of the Cr 3+ ions.

The growth and optical assessment of Cr3+-doped RX(BO3)4 crystals with R = Y, Gd; X = Al, Sc

Abstract This paper reports the high temperature top-seeded solution growth (TSSG) of Cr 3+ -doped mixed borate crystals having the general formula RX 3 (BO 3 ) 4 , with R 3+ = Y 3+ or Gd 3+ and X 3+ = Al 3+ or Sc 3+ . Both YAl 3 (BO 3 ) 4 (YAB) and GdAl 3 (BO 3 ) 4 (GAB) have been grown from a mixed K 2 Mo 3 O 10 B 2 O 3 flux, whereas YSc 3 (BO 3 ) 4 (YSB) and GdSc 3 (BO 3 ) 4 (GSB) are grown from K 2 Mo 3 O 10 Li 2 B 4 O 7 mixtures. Typical crystals ranging in volume from 60 mm 3 to 14 cm 3 have been grown, with Cr 3+ concentrations of order 0.2–5.0 at%. Measurements of the optical absorption and luminescence spectra show that in YAl 3 (BO 3 ) 4 and GdAl 3 (BO 3 ) 4 the Cr 3+ ions occupy strong crystal field sites whereas in YSc 3 (BO 3 ) 4 and GdSc 3 (BO 3 ) 4 these ions occupy weak field sites. The significance of this observation is that the Cr 3+ ions in weak field sites emit via the broadband 4 T 2 → 4 A 2 transitions, with obvious potential as tunable laser gain media.

Crystal growth and optical assessment of Nd3+ :GdAl3(BO3)4 crystal

This paper reports the high-temperature top-seeded solution growth (TSSG) of large single crystal of Nd3+-doped GdAl3(BO3)4 from a K2Mo3O10-LiF flux cooled at a rate of 2 K/day and using a crystal rotation rate of 4.5 rpm. Thepolarized optical absorption and fluorescence spectra of Nd3+ : GdAl3(BO3)4 crystal have been investigated. Polarized absorption cross sections are 4.3x10-20 and 2.6x10-20 cm2 with band widths of 8.7 and 6.1 nm for σ- and π-polarization, respectively. The emission cross sections are 6.3x10-19 and 6.0x10-19 cm2 for σ- and π-polarization, respectively, at 1062 nm corresponding to the 4F3/2→4I11/2 transition. The fluorescence lifetime is 54.3 and 57 μs at 300 and 20 K, respectively.

Optical spectroscopy of , and single crystals

The optical absorption and photoluminescence properties of , and crystals grown by laser-heated pedestal growth (LHPG) have been measured at T = 14 K, 77 K and 300 K. These spectra show very high absorption coefficients for the three rare-earth (RE) ions and strong luminescence signals characteristic of ions at high concentration in distorted symmetry sites. The rare-earth ions in these compounds are shown to occupy single sites only. The strong luminescence output at 300 K makes these crystals potential hosts for diode-pumped laser gain media.

The magnetic and optical properties of in

Electron spin-resonance (ESR) spectra of -doped (LiCAF) crystals measured at low temperature reveal three distinct centres in the crystal. The most intense spectrum is fitted to a spin Hamiltonian with trigonal symmetry and is associated with ions that substitute for ions in the LiCAF structure. Two other spectra have orthorhombic symmetry: they are assigned to ions with charge-compensating -ion vacancies near to the impurity ions. The and components of the polarized optical absorption and luminescence spectra of in LiCAF show there to be at least two centres. The intensities of optical transitions of are calculated using the eigenfunctions of the ground and excited states of in trigonal and orthorhombic symmetry and are compared with the observed polarizations of the absorption and luminescence.

Optical spectroscopy and optimal crystal growth of some Cr4+-doped garnets

This paper describes the growth of single crystals of Cr4+ -doped Y3 Ga5 O12 (YGG) and Y3 Al5 O12 (YAG). Control of melt composition and post-growth annealing yields material that contains optimal concentrations of Cr4+ ions in distorted tetrahedral sites normally occupied by Ga3+ ions in YGG and Al3+ ions in YAG. Both Cr4+ -doped garnets exhibit strong visible and near-infrared absorption bands with peak cross sections of order 10-18 -10-19 cm2 and emit into vibronically broadened but weakly allowed 3 B2 (3 T2 ) 3 B1 (3 A2 ) transitions. Nonradiative decay is more efficient in YGG than in YAG as a consequence of the slightly reduced energy gap against radiative decay in the Ga-based garnet.

Crystal growth and optical characterisation of novel 3d2 ion laser hosts

This paper reports the Czochralski growth of large single crystals of Cr4+ : Ca10(PO4)6F2 (CFAP), Mn5+ : Sr10(VO4)6F2 (SVAP), and Cr4+ : SrGdGa3O7 (SGGM) crystals in radio-frequency heated induction furnaces operating at temperatures in the range 1873–1923 K. Crystals having excellent optical quality were produced using growth rates of 2.8–11.2 μm/s and rotation rates between π/2 and π rad/s. The optical absorption spectra confirm that the 3d2-configuration ions reside at distorted (VO4), (PO4) and (GaO4) tetrahedra in SVAP, CFAP and SGGM, respectively, at which transition probabilities are enhanced by the odd-parity terms at the Cs site symmetry in the crystal field expansion. The near-infrared luminescence is broadened by structural disorder and is strongly dependent upon both the crystal field strength and temperature. Although the luminescence in Cr4+ : CFAP and Cr4+ : SGGM occur as broadbands in the near infrared region, the intensity is weak at 300 K as a consequence of nonradiative decay. In contrast, the effects of nonradiative decay in Mn5+ : SVAP are weak and strong zero-phonon lines are observed even at room temperature.