Structural Relaxation around Substitutional Cr³⁺ Ions in Sapphire

Abstract

The local structural arrangement of ions around a substitutional Cr3+ ion in sapphire (ruby) has been studied experimentally using extended X‐ray absorption fine structure (EXAFS) in the vicinity of the Cr absorption edge. The findings are compared with an ionic model (Mott‐Littleton) computation using two sets of pairwise potentials. Both the EXAFS results and the computations reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement similar to that for Cr in α‐Cr2O3. Also, most of the structural relaxation is accommodated by the first shell of oxygen and aluminum ions around the substituted Cr3+ ion. The computations also indicate that with applied pressure (tensile and compressive) the ionic positions change self‐similarly and in proportion to the macroscopic strain.

Optical Transitions in Ruby across the Corundum toRh2O3(II) Phase Transformation

“…Table I compares the atomic arrangement around chromium with those around aluminum in Al 2 O 3 and chromium in Cr 2 O 3 at zero pressure. The atomic environment of chromium in ruby is found to be intermediate between chromium's in Cr 2 O 3 and aluminum's in the host lattice, in agreement with experimental measurements [13][14][15]. Figure 1 displays two important trends which are valid up to the highest pressures investigated:…”

Structural relaxations around Ti, Cr and Fe impurities in α-Al2O3probed by x-ray absorption near-edge structure combined with first-principles calculations

“…The small displacement of chromium is in agreement with various experimental studies of the literature. Cr 3+ are slightly larger (+0.015Å) than those determined from EXAFS measurements on powder samples, 36 and in good agreement (+0.005Å) with EXAFS measurements performed and on the same single crystal as the one used in the present study. 3 They are also consistent with data extracted from optical spectroscopy.…”

Structural, optical and magnetic properties of Cr doped ZnO microrods prepared by spray pyrolysis method

“…It was seen that undoped and Cr-doped ZnO samples exhibit peaks corresponding to (100), (002), (101) and (102) indicates that the samples were in a state of strain and that along the c-axis this strain was compressive [22]. The reason for the change in this lattice parameter may be due to the concentration of foreign atoms and defects, and their difference of ionic radii with respect to the substituted matrix ion ions; since the radius of Cr 3+ ions (0.63 Å) is smaller than that the Zn 2+ ions (0.74 Å) and the Cr-O bond in Cr 2 O 3 is shorter than the Zn-O bond in ZnO (1.96 Å compared to 1.98 Å) [23,24] this is consistent both with the XRD data above and also with the XPS data below. The other possible reason for the change in lattice constant may be due to the differences in the linear thermal expansion coefficients of ZnO samples with different Cr molar fractions and soda-lime glass [25].…”