Mg-doped Crystals Research Articles

Formation of Grown-In Nitrogen Vacancies and Interstitials in Highly Mg-Doped Ammonothermal GaN.

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 1018 and 2 × 1019 cm-3 and resistivities of ~1011 Ωcm and ~106 Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed.

First-principles study on the electronic structures and optical properties of Mg-doped KDP crystal

First-principles method was employed to study the defect formation energy, electronic structure, and optical properties of Potassium Dihydrogen Phosphate (KDP) crystals doped with Mg in both paraelectric phase (PE-KDP) and ferroelectric phase (FE-KDP). Additionally, we employed the HSE hybrid density functional to address the issue of “band gap” and utilized the finite size correction-finite volume (FNV) method to rectify defect formation energies. Due to hybridization of Mg-3s orbitals with O-2p orbitals, a defect energy level is created at the conduction band minimum (CBM), resulting in the band gap of Mg-doped KDP crystals being smaller than that of perfect crystals. Moreover, the incorporation of Mg leads to significant lattice distortions and alters bond lengths, particularly the O–Mg bonds, which are stretched by 0.65–0.77 Å. The optical spectra also have been obtained under the consideration of electron-phonon coupling. The absorption peak locates at 4.37 eV (284 nm) in PE-KDP. The absorption peak locates at 4.66 eV (266 nm) in FE-KDP. Moreover, the significant Stokes shifts (3.67 eV and 2.9 eV) observed in Mg-doped KDP crystals were attributed to lattice relaxation energy and distortions induced by the substitution. The calculated results indicate that the quality of the KDP crystals may be reduce by introducing Mg-impurities. The work provides valuable insights into the optical characteristics of Mg-doped KDP crystals and their potential applications in laser technology.

Correlation between Infrared Absorption and Lithium Sublattice Disorder in Magnesium-Doped Lithium Niobate

Lithium niobate is a ferro- and piezoelectric material with excellent optical properties and a wide variety of applications. The defect structures of congruent and Mg-doped crystals are still under intense discussion. In this work, undoped lithium niobate and magnesium-doped lithium niobate grown from congruent melt with the addition of 0 to 9 mol% MgO were investigated by infrared absorption, establishing the dependence of the absorbance on the Mg-doping level in two bands related to OH- stretching vibrations. The absorption band at 3485 cm-1 peaks at a MgO concentration in melt of 1 mol% and vanishes for MgO concentrations above the threshold level for optical damage suppression (4.8 mol%). A corresponding peak occurs in the minimum yield of the 7Li(p,α)4He reaction during ion channeling measurements, indicating a maximum of disorder in the Li sublattice. A possible explanation for this correlation is the attribution of this absorption band to ilmenite stacking fault sequences instead of isolated NbLi antisites in undoped and low-doped material. On the other hand, the OH- absorption band at 3535 cm-1 stays weak up to the MgO concentration threshold, and then increases, hinting to a defect related to the increase of vacancies due to the lack of charge compensation.

Application of Electron-Cloud Equivalent Volume in Analyzing the Effect of Composition on Band Gap

In this paper, the variation in band gap E g with composition has been investigated by means of the parameter V e (V e is the electron-cloud equivalent volume of the outmost electron in a molecular). The reasoning indicates that E g is correlated with V e. The measured data of some alkaline earth metal halides support the reasoning. On this basis, with the help of the relationship between composition and V e, the correlation among composition, V e and E g is determined. Since the Nephelauxetic effect affects V e, it also affects E g, that is, from the perspective of parameter V e, the Nephelauxetic effect is a factor affecting E g. A simple method for analyzing the effect of composition on E g by means of the Nephelauxetic effect and the chemical bond property is proposed. For the alkaline earth metal halides, Mg-doped ZnO crystal, elements of group 4A in the periodic table and some phenomena, the effect of composition on E g is analyzed by using this method, and the analysis results are consistent with their realities, which verify the effectiveness of this method. This work helps to adjust E g by changing the composition and develop new materials efficiently.

Defect energy levels in monoclinic β-Ga2O3

The defect levels in semiconductor are crucial to the performance of related devices. However, the positions of the defect levels in monoclinic β-Ga2O3, such as that for oxygen vacancy (VO), Ga–O divacancy, and self-trapped hole (STH), are still controversial. In this paper, we systematically investigated the photoluminescence and photoluminescence excitation spectra of undoped, Si-doped, and Mg-doped β-Ga2O3 single crystals grown by the edge-defined film-fed growth method. It is found that the oxygen vacancies are deep donors and their levels are related to the coordinations of the vacancies. For each crystal, the recombination between VO and Ga–O divacancy levels is the origin of the blue emission, and the red emission originates from the transition from VO to STH levels. The location of STH level is also related to the geometrical position of oxygen. For the Si-doped crystal, no red emission is observed due to the annihilation of STHs. While a Mg impurity level is produced at ∼1 eV above the valence band maximum, which causes an extra red emission in the spectra of Mg-doped crystal. Our results strongly support the recent theoretical results obtained via first-principle calculations [Q. D. Ho et al., Phys. Rev. B 97 (2018) 115163] and could be benefit for designing β-Ga2O3 related devices.

Defect structure of near-stoichiometric Mg-doped LiNbO3 crystals prepared by different method

Mg-doped (0.5 mol% and 1 mol%) near-stoichiometric lithium niobate (nSLN) crystals were prepared by Li-rich growth method and VTE technique, respectively. Their defect structures were characterized by high resolution X-ray diffraction (HRXRD), OH– vibrational spectra, Raman spectra and ultraviolet-absorption spectra. The crystalline perfection analysis by HRXRD measurements reveals that diffraction curves of all Mg-doped nSLN crystals contain a single diffraction peak, signifying the absence of internal sub-grain boundary. In comparison to SLN-R crystals (grew by Li-rich growth method), the crystalline quality of SLN-V crystals (prepared by VTE technique) is better. The results indicate that MgNb3+ defect complexes exist in SLN-R crystals, even if Mg content is under the threshold, and that most of Mg2+ occupy Li site, extra low Mg2+ and the unavoidable trivalence impurities occupy Nb site in SLN-V crystals. The cation sublattice disorder of SLN-R crystals increases relative to SLN-V crystals. A remarkably weak OH– absorption band indicates that H+ content is extremely low in SLN-V crystals.

Comment on “Self-diffusion in high-purity α-Al2O3: Comparison of Ti-doped, Mg-doped and undoped single crystals” P. Fielitz, S. Ganschow, K. Klemens, and G. Borchardt, J. Eur. Ceram. Soc., 41, (2021), 663–668”

We comment on recent observations of O and Al self-diffusion in single-crystal sapphire with variable doping concentrations of Mg and Ti by Fielitz et al [1]. The paper reports a null effect of aliovalent doping on oxygen diffusivity. We posit that the extensive heat treatment involved in their experimental protocol may have caused dopant evaporation near the surface, and therefore the null result in oxygen diffusivity, whereas an effect on Al diffusivity is still discernible due to a greater diffusion depth of Al. We propose that buffering mechanisms are ultimately responsible for modest increases of self-diffusion with respect to dopant concentrations; in the Mg-doped case, DFT calculations suggest that negatively charged Mg interstitial defects are the principal charge compensating defects for positively charged Mg substitutional ions.

Anisotropic luminescence and third-order electric susceptibility of Mg-doped gallium oxide under the half-bandgap edge.

Strong anisotropy of photoluminescence of a (100)-cut β-Ga2O3 and a Mg-doped β-Ga2O3 single crystals was found in UV and visible spectral range, the bands of which were attributed to different types of transitions in the samples. Green photoluminescence in the Mg-doped sample was enhanced approximately twice. A remarkable enhancement of two-photon absorption and self-focusing in β-Ga2O3 after doping was revealed by 340-fs laser Z-scanning at 515 nm. The absolute value of complex third order susceptibility χ(3) determined from the study increases by 19 times in [001] lattice direction. Saturable absorption and associated self-defocusing were found in the undoped crystal in the [010] direction, which was explained by the anisotropic excitation of F-centers on intrinsic oxygen defects. This effect falls out of resonance in the Mg-doped crystal. The χ(3) values which are provided by a decrease of bandgap in Mg-doped β-Ga2O3 are χ(3) [001] = 1.85·10-12 esu and χ(3) [010]=χ(3)yyyy = 0.92·10-12 esu. Our result is only one order of magnitude lower than the best characteristic in green demonstrated by a Mg-doped GaN, which encourages subsequent development of Mg-doped β-Ga2O3 as an effective nonlinear optical material in this region.

Charge trapping at Fe due to midgap levels in Ga2O3

Fe acts as an electron trap in gallium oxide (Ga2O3), thereby producing a semi-insulating material that can be used in device fabrication. However, such trapping can lead to negative effects when Fe is unintentionally incorporated into bulk crystals or thin films. In this work, photoinduced electron paramagnetic resonance (photo-EPR) is used to investigate carrier capture at Fe in β-Ga2O3. Two crystals doped with 8 × 1017 cm−3 and 5 × 1018 cm−3 Fe and one Mg-doped crystal containing 7 × 1016 cm−3 unintentional Fe are studied by illuminating with LEDs of photon energies 0.7–4.7 eV. Steady state photo-EPR results show that electrons excited from Ir, an unintentional impurity in bulk crystals, are trapped at Fe during illumination with photon energy greater than 2 eV. Significantly, however, trapping at Fe also occurs in the crystals where Ir does not participate. In such cases, we suggest that excitation of intrinsic defects such as oxygen or gallium vacancies are responsible for trapping of carriers at Fe. The results imply that the investigation of intrinsic defects and their interaction with Fe is necessary to realize stable and reliable Ga2O3:Fe devices.

Growth and characterization of pure and metals-doped organic nonlinear optical single crystal: l-alanine alaninium nitrate (LAAN)

The pure and metal ion (Cu2+, Zn2+ and Mg2+)-doped LAAN single crystal was developed by slow evaporation technique under ambient temperature. Cell parameters of the grown crystal were identified using single crystal X-ray diffraction analysis and found that the material crystallizes in orthorhombic crystal system with space group of P21.The presence of functional groups and the spectral properties were assessed by FTIR analysis. Transmittance spectrum reveals that the crystal has a low UV cut-off of 360 nm and has a good transmittance in the entire visible region. The band gap energy of LAAN and doped (Zn, Cu and Mg) LAAN crystals were estimated to be 3.38, 3.22, 2.75, and 3.13 eV by Tauc plot. Prevention of the recombination process of electron–hole pair was also confirmed by photoluminescence spectra analysis. The dielectric constant of pure crystal was found to be less than that of doped crystals. The AC conductivity was found to increase after doping and with the increase in temperature. Second harmonic generation efficiency of the grown crystal has been measured by Kurtz and Perry technique and it was found that Cu-doped LAAN is 3.2 times higher than that of KDP and 2.1, 1.5, and 1.8 times more prominent than pure, Zn, and Mg-doped LAAN crystals, respectively.

Effect of malachite green dye doping in tristhiourea zinc (II) sulphate single crystal—a potential nonlinear optical material

Tristhiourea zinc (II) sulphate (ZTS) is a semi-organic and highly nonlinear optical material used for second harmonic generation (SHG) and other nonlinear optical (NLO) applications. The good quality of pure ZTS (PZTS) and 0.05 mol% and 0.1 mol% malachite green (MG) dye-doped ZTS crystals were harvested by employing low cost effective slow evaporation solution growth technique. The as-grown crystals were subjected to different characterization techniques to see the effect of MG doping on structural, mechanical, thermal, optical and NLO properties. The sharp and intense peaks observed in powder X-ray diffraction and high-resolution X-ray diffraction of PZTS and doped ZTS confirmed the good crystalline quality of pure and doped ZTS. A comparison of FT-IR spectroscopy studies of pure and doped crystals confirmed the incorporation of MG dopant in ZTS from the observation of shifting of various absorption bands. The optical bandgap energies and cut off wavelengths for pure and doped ZTS crystals were studied using UV–Vis.-NIR spectroscopy over an extended spectral range of 200–1100 nm at room temperature. The photoluminescence study revealed that microscopic defects like vacancies and self interstitials are reduced. Thermal stability of the grown crystals were studied by thermogravimetric analysis and found that the thermal stability is not affected much by doping. Vicker’s hardness test was employed to pure and MG-doped ZTS crystals and elucidated that the doped ZTS crystals were found to be slightly softer than pure ZTS. The frequency dependent dielectric constants i.e. ɛr (real), ei (imaginary), dielectric loss (tan δ) and ac electrical conductivity (σac) of pure and doped ZTS crystals were studied by subjecting the grown crystals with impedance analyzer. To observe the NLO property, the grown crystals were subjected to Kurtz powder technique. The results of SHG studies expounded that the SHG efficiency was increased by 2.75 times with MG doping in ZTS crystals.

Deep donor behavior of iron in β-Ga2O3 crystals: Establishing the Fe4+/3+ level

The Fe4+/3+ donor level is experimentally determined to be 0.70 eV (±0.05 eV) above the valence band maximum in β-Ga2O3. Electron paramagnetic resonance (EPR) is used to monitor Fe3+ ions that are unintentionally present in an Mg-doped β-Ga2O3 crystal (with a low Fermi level). For temperatures near 255 K, exposure to 325 nm laser light converts a portion of the Fe3+ ions to Fe4+ and Fe2+ ions and, at the same time, forms neutral magnesium acceptors (MgGa0) and neutral Ir donors (Ir3+). After removing the light, the intensity of the Fe3+ EPR spectrum has a significant additional decrease as holes thermally released to the valence band from rapidly decaying neutral Mg acceptors are trapped at Fe3+ ions and form even more Fe4+ ions. This demonstrates that the Mg0/− acceptor level, near 0.65 eV, is closer to the valence band than the Fe4+/3+ level. Following the fast initial post-light decrease, the Fe3+ spectrum then slowly recovers as Fe4+ ions are destroyed by electrons thermally excited from the valence band. An activation energy for the thermal decay of the Fe4+ donors, and thus a value for the Fe4+/3+ level, is obtained from the analysis of five Fe3+ isothermal recovery curves taken from the Mg-doped crystal between 250 and 270 K. A first-order kinetics model is used, as minimal retrapping is observed. In separate experiments, EPR shows that Fe4+ ions are also produced in an Fe-doped β-Ga2O3 crystal (without Mg acceptors) during exposures to laser light at temperatures near 255 K.

Self-diffusion in high-purity α-Al2O3: Comparison of Ti-doped, Mg-doped and undoped single crystals

Oxygen diffusivity in α-alumina is characterised by a scatter band of about one decade and a half in an Arrhenius diagram. In order to control decisive material parameters the present study is based on high-purity single crystals which were deliberately doped with aliovalent elements (Mg or Ti) and simultaneously annealed in an 18O2 atmosphere (200 mbar at 1650 °C). In such a crucial experiment the diffusivities in undoped and acceptor or donor doped alumina are identical within a factor of about 2. Contradictory findings in the past on the impact of Mg and Ti on the oxygen diffusion can be rationalised due to published similar calculated energies for the most probable dopant incorporation reactions. Al self-diffusion is much more rapid than oxygen diffusion in undoped and Ti-doped α-alumina. In this work we show for the first time that Mg doping drastically reduces the Al diffusivity in α-alumina.

Superposed picosecond luminescence kinetics in lithium niobate revealed by means of broadband fs-fluorescence upconversion spectroscopy

Various manifestations of small polarons strongly affect the linear and nonlinear optical properties of the oxide crystal lithium niobate (hbox {LiNbO}_3, LN). While related transient absorption phenomena in LN have been extensively studied in recent decades, a sound microscopic picture describing the blue-green (photo)luminescence of lithium niobate single crystals is still missing. In particular, almost nothing is known about: (i) the luminescence build-up and (ii) its room temperature decay. We present here the results of our systematic experimental study using nominally undoped and Mg-doped LN crystals with different Mg concentration. Picosecond luminescence was detected by means of femtosecond fluorescence upconversion spectroscopy (FLUPS) extended to the inspection of oxide crystals in reflection geometry. Two distinct luminescence decay components on the picosecond time scale are revealed. While a short exponential decay is present in each sample, a longer non-exponential decay clearly depends on the crystal composition. Since transient absorption spectroscopy excludes geminate small polaron annihilation as microscopic cause of the luminescence, both decay components are discussed in the context of self-trapped exciton (STE) transport and decay.

Synthesis, structure and properties of layered Pr2MoO6-based oxymolybdates doped with Mg.

Undoped and Mg-doped Pr2MoO6 oxymolybdate polycrystals and single crystals have been prepared by solid-state reactions and flux growth. The compounds have been characterized by powder X-ray diffraction, energy-dispersive spectroscopy, inductively coupled plasma mass spectrometry, scanning transmission electron microscopy, single crystal X-ray structure analysis, differential scanning calorimetry and thermogravimetry. The (MgO)x(Pr2O3)y(MoO3)z (x + y + z = 1) solid solution series has been shown to extend to x = 0.03. The structure of the Mg-doped Pr2MoO6 single crystals can be represented as superimposed lattices of the main matrix (Pr2MoO6) and lattices in which Mo atoms are partially replaced by Mg. The incorporation of Mg atoms into the structure of Pr2MoO6 results in the disordering of the praseodymium and oxygen lattices. Both the polycrystalline and single-crystal Mg-doped samples are hygroscopic.

Effect of Mg(II) and Na(I) Doping on the Electronic Structure and Mechanical Properties of Kaolinite

Because kaolinite has multiple defects, it is very important to study the effect of different doped cations on the electronic structure and mechanical properties of kaolinite (Al4Si4O18H8) from the microscopic point of view with the first-principle calculation method. The results exhibited that the doping of Mg(II) and Na(I) makes the ion bond and layer spacing of kaolinite crystal change, and the bond length of the chemical bond between the doped and O atom is positively related to the atomic radius of the doped cations. Compared with undoped kaolinite crystal, the band gap width of the Mg-doped and Na-doped kaolinite crystal was larger, but the typical insulator characteristics were still maintained. Compared with undoped kaolinite crystal, Mg-doped and Na-doped kaolinite crystal had more electron transfer to O, while the Mg–O bond and Na–O bond had more ionic bond properties and less covalent bond composition than the Al–O bond. Finally, the elastic properties of undoped, Mg-doped, and Na-doped kaolinite crystal were further analyzed by calculating the elastic constant matrix. The influence of doping Mg(II) and Na(I) on C11 and C22 was greater than that on C33, indicating that doping had a greater influence on the stiffness in the direction of the parallel crystal plane. The doping of Mg(II) and Na(I) weakened the rigidity of kaolinite crystal materials and improved the plasticity and ductility of the materials. The atom-scale information provided a basis for explaining the mechanical behavior of kaolinite and is expected to provide guidance for solving the deformation problems in soft rock roadways.

Experimental determination of the (0/−) level for Mg acceptors in β -Ga2O3 crystals

Electron paramagnetic resonance (EPR) is used to experimentally determine the (0/−) level of the Mg acceptor in an Mg-doped β-Ga2O3 crystal. Our results place this level 0.65 eV (±0.05 eV) above the valence band, a position closer to the valence band than the predictions of several recent computational studies. The crystal used in this investigation was grown by the Czochralski method and contains large concentrations of Mg acceptors and Ir donors, as well as a small concentration of Fe ions and an even smaller concentration of Cr ions. Below room temperature, illumination with 325 nm laser light produces the characteristic EPR spectrum from neutral Mg acceptors (MgGa0). A portion of the singly ionized Ir4+ donors are converted to their neutral Ir3+ state at the same time. For temperatures near 250 K, the photoinduced EPR spectrum from the neutral MgGa0 acceptors begins to decay immediately after the laser light is removed, as electrons are thermally excited from the valence band to the Mg acceptor. Holes left in the valence band recombine with electrons at the deeper Ir3+ ions and restore the Ir4+ ions. An activation energy for the thermal decay of the MgGa0 acceptors, and thus a value for the (0/−) level, is obtained by using a general-order kinetics model to analyze a set of five isothermal decay curves taken at temperatures between 240 and 260 K.

Ir4+ ions in β-Ga2O3 crystals: An unintentional deep donor

Electron paramagnetic resonance (EPR) and infrared absorption are used to detect Ir4+ ions in β-Ga2O3 crystals. Mg and Fe doped crystals are investigated, and concentrations of Ir4+ ions greater than 1 × 1018 cm−3 are observed. The source of the unintentional deep iridium donors is the crucible used to grow the crystal. In the Mg-doped crystals, the Ir4+ ions provide compensation for the singly ionized Mg acceptors and thus contribute to the difficulties in producing p-type behavior. The Ir4+ ions replace Ga3+ ions at the Ga(2) sites, with the six oxygen neighbors forming a distorted octahedron. A large spin-orbit coupling causes these Ir4+ ions to have a low-spin (5d5, S = 1/2) ground state. The EPR spectrum consists of one broad line with a significant angular dependence. Principal values of the g matrix are 2.662, 1.815, and 0.541 (with principal axes near the crystal a, b, and c directions, respectively). Ionizing radiation at 77 K decreases the Ir4+ EPR signal in Mg-doped crystals and increases the signal in Fe-doped crystals. In addition to the EPR spectrum, the Ir4+ ions have an infrared absorption band representing a d-d transition within the t2g orbitals. At room temperature, this band peaks near 5153 cm−1 (1.94 μm) and has a width of 17 cm−1. The band is highly polarized: its intensity is maximum when the electric field E is parallel to the b direction in the crystal and is nearly zero when E is along the c direction.

A comprehensive investigation on core optoelectronic and laser properties of ZTS single crystals: an effect of Mg2+ doping

The synthesis and large-sized single crystals of pure and Mg2+-doped zinc (tris) thiourea sulfate has been developed using slow evaporation methods at 300 K. The size of the grown crystal for pure, 2% Mg and 5% Mg-doped ZTS is found to be ~ 15 mm × 17 mm, ~ 20 mm × 18 mm, and 22 mm × 10 mm, respectively. The crystal structural and vibrational modes are identified by powder X-ray diffraction, FT-IR and FT-Raman analyses. The grown crystal with 5% Mg doping possesses higher optical transparency (67%). Optical energy band gap is found ~ 4.36 eV for 2% Mg and ~ 4.41 eV for 5% Mg-doped crystals. Enhancement in PL intensity of UV band was observed due to doping. The third-order nonlinear susceptibility, χ3 is found to be enhanced due to doping which is in the order of 10−3 esu. DSC study confirms that the 5% MgZTS crystals possess higher thermal stability than pure as well as 2% MgZTS. Furthermore, the dielectric study confirms that the grown crystals possess low defects. The microhardness is also found to be enhanced due to doping. All the properties of ZTS are found to be enhanced by Mg doping and make it more suitable for optoelectronic and nonlinear applications compared to pure.

Electron paramagnetic resonance study of neutral Mg acceptors in β-Ga2O3 crystals

Electron paramagnetic resonance (EPR) is used to directly observe and characterize neutral Mg acceptors (MgGa0) in a β-Ga2O3 crystal. These acceptors, best considered as small polarons, are produced when the Mg-doped crystal is irradiated at or near 77 K with x rays. During the irradiation, neutral acceptors are formed when holes are trapped at singly ionized Mg acceptors (MgGa−). Unintentionally present Fe3+ (3d5) and Cr3+ (3d3) transition-metal ions serve as the corresponding electron traps. The hole is localized in a nonbonding p orbital on a threefold-coordinated oxygen ion adjacent to an Mg ion at a sixfold-coordinated Ga site. These MgGa0 acceptors (S = 1/2) have a slightly anisotropic g matrix (principal values are 2.0038, 2.0153, and 2.0371). There is also partially resolved 69Ga and 71Ga hyperfine structure resulting from unequal interactions with the two Ga ions adjacent to the hole. With the magnetic field along the a direction, hyperfine parameters are 2.61 and 1.18 mT for the 69Ga nuclei at the two inequivalent neighboring Ga sites. The MgGa0 acceptors thermally convert back to their nonparamagnetic MgGa− charge state when the temperature of the crystal is raised above approximately 250 K.