Extended investigations on the pressure stability of AlB₄O₆N:Cr³⁺

1 Introduction

Research under static ultra-high pressure can be carried out in diamond anvil cells (DAC), which consist of two opposing diamonds with flattened tips, allowing samples to be squeezed between them. ¹ DACs are crucial in studying material properties under extreme conditions, offering insights into phase transitions, chemical reactions, and material behavior under immense pressure. ¹ The tremendous opportunities offered by diamond anvil cells were shown by Dubrovinskaia & Dubrovinsky et al. in 2016, who investigated materials at the (to the best of our knowledge) highest ever reached pressure of 1 TPa with DACs in a special setup. ² Pressure inside the DAC has to be estimated by calibration, for which the ruby fluorescence method has become the mainly performed method for decades ¹ and still is the state-of-the-art calibrating technique in DACs up to a pressure of ca. 100 GPa. ^{3 , 4 , 5} This calibration method was introduced in 1972 by Forman et al. ⁶ and is based on a shift of the R lines of Cr³⁺-doped Al₂O₃ under hydrostatic pressure and a broadening of these lines if the ruby experiences nonhydrostatic stresses. ⁶ Further optimizations by Barnett et al. made this technique very rapid, convenient, and applicable for routine pressure measurements. ⁷ A tiny piece of ruby is placed inside the DAC along with the sample, and the luminescence is excited by strong light (e.g. a laser). ¹ The resulting R lines are usually at around 693 nm (ambient pressure) with an energy separation of only around 29 cm⁻¹. ⁸ However, this close proximity of these two R lines restricts its technical utility as reference material for pressure calibration at elevated pressures, because of broadening and merging of the lines. Therefore, any enhancement of these luminescence characteristics while maintaining the benefits of high brightness at high temperature and pressure conditions for Cr³⁺ in a stable inorganic host material would be highly desirable. In 2024 Widmann et al. developed the promising compound AlB₄O₆N:Cr³⁺ (see Figure 1), ⁹ which revealed extraordinary luminescence properties, low thermal expansion, high thermal stability and hardness suitable for multifunctional applications. In particular, AlB₄O₆N:Cr³⁺ features an almost undistorted octahedral symmetry of the Al³⁺ sites, which leads to a single R line at 683 nm in contrast to the two R lines of ruby. Up to now, AlB₄O₆N:Cr³⁺ was investigated up to a pressure of 52 GPa and fulfilled in this pressure range the main criteria for a possible luminescent pressure sensor for more precise pressure measurements based on the presence of one single R line.

Figure 1:

Crystal structure of MB₄O₆N (M = Al, Al/Cr, Cr) ^{9 , 10} in viewing direction [ 1 ‾ 00]. As representative, one [N(BO₃)₄] building unit is colored in red, which is crystallographically identical to the green ones. Tetrahedra with a B1 atom in their centers are colored in dark green, those with a B2 in mint.

Due to these promising results, we decided to extend the investigations of AlB₄O₆N:Cr³⁺ to pressures above 52 GPa. The new ultra-high pressure results were achieved with fluorescence measurements of AlB₄O₆N:Cr³⁺ and ruby inside a diamond anvil cell, complemented by single-crystal measurements (synchrotron data) at elevated pressures to reveal possible structural changes. The published high-pressure/high-temperature approach for the synthesis of AlB₄O₆N:Cr³⁺ was greatly improved as also reported in this paper.

2 Experimental procedures

2.2 X-ray powder diffraction analysis

The reaction product was analyzed with a STOE Stadi P powder diffractometer (STOE & Cie GmbH, Darmstadt, Germany), which was equipped with a Mythen 1K microstrip detector (Dectris, Baden-Daettwil, Switzerland) and a Ge(111) monochromator. The measurement was performed with MoKα ₁ radiation (λ = 0.7093 Å) in transmission geometry with a flat sample holder across a 2θ range of 2.0–60.5°. A Rietveld refinement ¹⁴ was performed with the program Topas 4.2 ¹⁵ (see Figure 2). Lattice parameters of a = 5.0378(2) and c = 8.2401(3) Å were determined, which are in accordance with the single-crystal values a = 5.0506(2) and c = 8.2497(3) Å for Al_0.97Cr_0.03B₄O₆N available in the literature. ⁹ Additional reflections could be explained by 18 wt% of the side phase NH₄B₅O₆(OH)₄·2H₂O. ¹⁶ In comparison to our first synthesis attempt, the ratio of the title compound and side phases (depending on the synthesis approach h-BN, ¹⁷ c-BN ¹⁸ and HP-NH₄B₃O₅ ¹⁹ ) is in the same range, but now the amount of product could be clearly increased. In the previous variant, a lot of product had to be removed in the de-assembling process. ⁹

Figure 2:

Rietveld refinement of the powder X-ray diffraction pattern (MoKα ₁ radiation) of the sample with 82 wt% AlB₄O₆N:Cr^{3+ 9} and 18 wt% of the additional phase NH₄B₅O₆(OH)₄·2H₂O. ¹⁶

3 Results and discussion

Pressure dependent measurements of the spectral position of the R line of AlB₄O₆N:Cr³⁺ (up to about 50(1) GPa) revealed a red shift with increasing pressure. ⁹ The structure analysis was performed at 52(1) GPa (single-crystal synchrotron data; inherent data is reported in this work) and showed a significant decrease of all the lattice parameters (see Table 1) and no occurrence of any phase transitions or decompositions. ⁹ In the following we discuss the results at even higher pressure.

After loading the sample into the DAC (before gas loading), the position of the single fluorescence line was measured at a wavelength of 683.7(2) nm, which is in good agreement with previous data ⁹ (Figure 3). Upon pressure increase in several steps, the line remained single up to 53(1) GPa and its position shifted linearly with dL/dp = 0.302(4) nm/GPa in accordance with our earlier results ⁹ (see Figure 3). Upon further pressure increase to 59(1) GPa, the fluorescence peak became asymmetric and clearly split into two well resolved lines upon compression above ∼74 GPa (see Figure 3).

Figure 3:

Examples of fluorescence spectra of AlB₄O₆N:Cr³⁺ (0.7 mol% Cr³⁺) collected upon compression using the pressure transmitting medium neon. Standard deviations are specified in parentheses.

To find out the reason for the splitting, a set of diffraction data of the single crystal of the title compound inside the diamond anvil cell was measured at a synchrotron under a pressure of 78(1) GPa. This is the highest pressure at which fluorescence was collected and the splitting of the R line was already very obvious (see Figure 3). The measurement basically revealed the same structure which is also found in the literature. ⁹ No phase transition or decomposition occurred (see Table 2), the main difference being smaller cell parameters (see Table 1) and bond lengths (see Tables 3 and 4), due to the increased pressure, as expected. As described by Widmann et al., ⁹ the splitting of the R line of Cr³⁺-based luminescence in ruby is caused by a distortion at the octahedrally coordinated Al/Cr sites. In contrast, AlB₄O₆N:Cr³⁺ shows one single R line at ambient pressure, because of nearly perfectly symmetrical [AlO₆] and [CrO₆] octahedra, whereas at 78(1) GPa a distortion seems to occur. Based on the bond lengths (see Tables 3 and 4), the difference between the M–O1 and M–O2 distances appears slightly increased, though this is still within the standard deviations. The bond angles do not show a significant change (see Table 5). Hence, a distortion could not be proven with certainty, but the single-crystal diffraction data revealed no other change in the structure and so a slight distortion within the standard deviation is the most likely explanation for the splitting of the R line. It should be mentioned that the accuracy of the single-crystal diffraction measurements at 78(1) GPa is quite moderate, which is expected for a material consisting of light elements compressed beyond ∼75 GPa.

In contrast to ruby, which has two R lines, AlB₄O₆N:Cr^{3+ 9} has the advantage that the splitting of its single R line initially occurs at a pressure higher than 59(1) GPa and the peaks of the separated R lines can be clearly distinguished up to at least 78(1) GPa, where we terminated our measurements. Future investigations will explore the maximum pressure at which AlB₄O₆N:Cr³⁺ can be compressed before its structure collapses.

4 Conclusions

The compound AlB₄O₆N:Cr³⁺ has recently been prepared and characterized. It showed extraordinary luminescence properties, low thermal expansion, high thermal stability and hardness, which are all suitable for multifunctional applications. ⁹ Furthermore, it was shown that it fulfills the main criteria of a possible alternative luminescent pressure sensor up to at least 52 GPa, enabling more precise pressure measurements on the basis of one single R line. ⁹ Due to these intriguing findings, we extended these measurements to a pressure of 78(1) GPa, which revealed a splitting of the initial single R line into two separate peaks, starting above 60(1) GPa. A single-crystal structure analysis at 78(1) GPa (synchrotron data) indicated a slight distortion of the [(Al/Cr)O₆] octahedra (within the standard deviations) and revealed that no decomposition occurs under the given extreme conditions.