---
title: LaGaO3:Eu3+ Phosphors in Luminescence Thermometry
url: https://www.emergentmind.com/topics/lagao3-eu3-phosphors
type: topic
---

# LaGaO3:Eu3+ Phosphors in Luminescence Thermometry

LaGaO\(_3\):Eu\(^ {3+}\) phosphors are phase-transition-based luminescent materials in which Eu\(^ {3+}\) emission is used both as a spectroscopic probe of structural symmetry and as a thermometric readout. In the reported system, Eu\(^ {3+}\) substitutes La\(^ {3+}\) in LaGaO\(_3\), whose host lattice undergoes a reversible first-order structural transformation from orthorhombic \(Pbnm\) to rhombohedral \(R3c\). The associated change in local symmetry modifies the relative intensities, Stark structure, and lifetime of the Eu\(^ {3+}\) \(^{5}D_0 \rightarrow {}^{7}F_J\) transitions, enabling ratiometric luminescence thermometry. Recent work has emphasized two engineering handles for this material class: microstructural control through synthesis route, and phase-transition tuning through partial substitution of Ga\(^ {3+}\) by Al\(^ {3+}\) and Sc\(^ {3+}\), thereby extending the usable temperature window while reducing hysteresis and increasing relative thermal sensitivity [2507.13750].

## 1. Crystal chemistry and structural transition

LaGaO\(_3\) exhibits a reversible first-order phase transition from orthorhombic \(Pbnm\) (space group No. 62; point symmetry \(C_s\) at the La site) to rhombohedral \(R3c\) (No. 167; point symmetry \(D_3\) at the La site). In undoped material this occurs at approximately \(430\)–\(440\) K. For LaGaO\(_3\):0.25%Eu\(^ {3+}\), differential scanning calorimetry gives \(T_c = 430.1\) K for the solid-state sample and a DSC peak at \(432.28\) K for the Pechini-derived sample; the first-order nature is evidenced by hysteresis in both DSC and the luminescent intensity ratio [2507.13750].

In this host, Eu\(^ {3+}\) substitutes La\(^ {3+}\) and is coordinated by \(8\)–\(12\) O\(^ {2-}\) ions in distorted polyhedra, whereas Ga\(^ {3+}\) occupies octahedral sites with coordination number \(6\). The co-dopants Al\(^ {3+}\) and Sc\(^ {3+}\) partially substitute Ga\(^ {3+}\), thus acting in the second coordination sphere relative to Eu\(^ {3+}\). The reported Shannon radii for coordination number \(6\) are \(R(\mathrm{Ga}^{3+}, VI)=0.62\) Å, \(R(\mathrm{Al}^{3+}, VI)=0.535\) Å, and \(R(\mathrm{Sc}^{3+}, VI)=0.745\) Å. This site selectivity is central because it allows substantial modification of the host-lattice instability without directly altering the immediate Eu\(^ {3+}\) coordination shell.

The compositional range investigated comprises Al\(^ {3+}\) contents of \(5\), \(10\), and \(15\) mol%, Sc\(^ {3+}\) content of \(2\) mol%, and co-doped sets containing \(10\%\) Al\(^ {3+}\) with \(2\%\), \(5\%\), or \(10\%\) Sc\(^ {3+}\). Within this space, the transition temperature is shifted from \(165\) K for 15% Al\(^ {3+}\) to \(491\) K for 2% Sc\(^ {3+}\), while the main text also reports approximately \(180\) K and \(500\) K for these endpoints. The coexistence of these values indicates a small internal discrepancy in reporting, but the qualitative result is unambiguous: the phase transition can be displaced across a wide interval by B-site substitution.

A recurrent misconception is that such co-doping necessarily perturbs the Eu\(^ {3+}\) spectroscopic center directly. In the reported system, the opposite conclusion is supported at low temperature: Al\(^ {3+}\)/Sc\(^ {3+}\) substitution is used precisely because it operates primarily through the second coordination sphere, decoupling phase-transition engineering from low-temperature Eu\(^ {3+}\) local symmetry to first order.

## 2. Synthesis, phase purity, and microstructure

Two synthetic routes were compared. The solid-state route used La\(_2\)O\(_3\) (99.999%), Ga\(_2\)O\(_3\) (99.999%), Eu\(_2\)O\(_3\) (99.99%), Al\(_2\)O\(_3\) (99.995%), and Sc\(_2\)O\(_3\) (99.99%), with dry mixing and grinding in hexane for 30 min, pre-annealing at 873 K for 3 h at 10 K min\(^{-1}\), regrinding, sintering at 1673 K for 6 h at 10 K min\(^{-1}\), and final grinding in air. The modified Pechini route used La\(_2\)O\(_3\) and Eu\(_2\)O\(_3\) converted to nitrates by dissolution in water with nitric acid and recrystallization, together with Ga(NO\(_3\))\(_3\cdot 9\)H\(_2\)O, citric acid, and PEG-200 at CA:metal = 6:1 and CA:PEG = 1:1, followed by drying at 363 K for 3 days to form a resin and then the same 873 K/3 h and 1673 K/6 h thermal treatment in air [2507.13750].

Powder X-ray diffraction showed single-phase LaGaO\(_3\) for all examined co-doping levels, including 0.25% Eu\(^ {3+}\), up to 15% Al\(^ {3+}\), up to 2% Sc\(^ {3+}\), and their combinations. Peak shifts track the expected unit-cell contraction for Al\(^ {3+}\) substitution and expansion for Sc\(^ {3+}\) substitution. For 10% and 15% Al\(^ {3+}\), split reflections near \(32.6^\circ\) indicate partial stabilization of the high-temperature \(R3c\) phase already at room temperature.

| Route | Mean grain size | DSC heating FWHM |
|---|---:|---:|
| Pechini | 272 nm | 18.18 K |
| Solid-state | 634 nm | 4.68 K |

SEM measurements on 100 grains showed that the Pechini sample is strongly aggregated and has broader grain-size dispersion, whereas the solid-state sample consists of larger grains with narrower dispersion. The corresponding DSC peak widths demonstrate that the macroscopic phase transition is much sharper in the more uniform solid-state material. This microstructural effect is not incidental: the reported interpretation is that a narrower grain-size distribution reduces the spread of local transition temperatures across the particle population, thereby steepening the collective spectral response and narrowing the hysteresis loop.

This establishes a key design principle for LaGaO\(_3\):Eu\(^ {3+}\): thermometric performance is not solely dictated by crystal chemistry. It is also controlled by mesoscale population statistics, especially grain-size dispersion.

## 3. Eu\(^ {3+}\) luminescence and ratiometric observables

The Eu\(^ {3+}\) emission spectrum is dominated by \(^{5}D_0 \rightarrow {}^{7}F_J\) transitions with bands centered at approximately 575 nm (\(J=0\)), 590 nm (\(J=1\)), 620 nm (\(J=2\)), 650 nm (\(J=3\)), and 700 nm (\(J=4\)). The \(^{5}D_0 \rightarrow {}^{7}F_1\) band is characterized as magnetic dipole and symmetry-insensitive, whereas \(^{5}D_0 \rightarrow {}^{7}F_2\) is electric dipole and hypersensitive to local symmetry [2507.13750].

Across the orthorhombic-to-rhombohedral transition, the relative intensity of \(^{5}D_0 \rightarrow {}^{7}F_2\) increases with respect to \(^{5}D_0 \rightarrow {}^{7}F_1\), while the \(^{5}D_0 \rightarrow {}^{7}F_4\) intensity decreases relative to \(^{5}D_0 \rightarrow {}^{7}F_2\). Simultaneously, the Stark structure simplifies in the high-temperature phase, consistent with the increase in site symmetry from \(C_s\) to \(D_3\). Peaks shift modestly and band shapes change. The \(^{5}D_0\) lifetime decreases from 1.06 ms in the low-temperature phase to 0.81 ms in the high-temperature phase, indicating altered radiative and nonradiative balance through the transition.

The principal thermometric observable is a luminescence intensity ratio,
\[
\mathrm{LIR}_1(T)=\frac{I(^{5}D_0\rightarrow{}^{7}F_2)}{I(^{5}D_0\rightarrow{}^{7}F_1)},
\]
implemented in practice through selected Stark components in wavelength windows around the \(^{7}F_2\) and \(^{7}F_1\) regions. Because the ratio is driven by symmetry-sensitive spectral redistribution rather than absolute intensity alone, it directly reflects the progress of the structural transition.

A second ratio was used to test whether co-doping alters the Eu\(^ {3+}\) site at low temperature:
\[
\mathrm{LIR}_2=
\frac{\int_{600\,\mathrm{nm}}^{630\,\mathrm{nm}} I(\lambda)\,\mathrm{d}\lambda}
{\int_{580\,\mathrm{nm}}^{600\,\mathrm{nm}} I(\lambda)\,\mathrm{d}\lambda}.
\]
Across the Al\(^ {3+}\)/Sc\(^ {3+}\) series, \(\mathrm{LIR}_2 \approx 2.6\text{–}2.8\) at 83 K. Low-temperature decay profiles at 83 K are likewise nearly identical among compositions. Together, these observations support the claim that co-doping shifts the host transition while leaving the low-temperature Eu\(^ {3+}\) spectroscopic environment essentially unchanged.

## 4. Thermometric performance and hysteresis

The relative thermal sensitivity is defined as
\[
S_r(T)=\frac{1}{\mathrm{LIR}_1(T)}\frac{\mathrm{d}\,\mathrm{LIR}_1}{\mathrm{d}T}\times 100\%\,\mathrm{K}^{-1}.
\]
For LaGaO\(_3\):0.25%Eu\(^ {3+}\), the Pechini sample reaches \(S_{r,\max}=3.0\%\ \mathrm{K}^{-1}\) at approximately 460 K, whereas the solid-state sample reaches \(S_{r,\max}=18.2\%\ \mathrm{K}^{-1}\) at approximately 420 K. The abrupt increase in \(\mathrm{LIR}_1\) begins near 450 K for the Pechini material and near 410 K for the solid-state material, mirroring the broader and narrower DSC features, respectively [2507.13750].

The thermometric limitation intrinsic to a first-order transition is hysteresis. It was quantified using
\[
\Delta \mathrm{LIR}_1(T)=\mathrm{LIR}_1^{(\mathrm{cooling})}(T)-\mathrm{LIR}_1^{(\mathrm{heating})}(T).
\]
The maximal hysteresis in the luminescent ratio decreases from approximately 1.15 in the Pechini sample to 0.5 in the solid-state sample. The corresponding temperature uncertainty \(S_T\), defined operationally as the maximal temperature error caused by using a single-valued calibration in the presence of hysteresis, decreases from approximately 5.8 K to approximately 1.2 K.

These data directly refute the assumption that sensitivity and hysteresis must trade off unfavorably in phase-transition-based thermometers. In this system, microstructural optimization improves both figures of merit simultaneously: the sharper transition produces a larger \(\mathrm{dLIR}_1/\mathrm{d}T\), while the narrower transition distribution reduces loop width.

For applications that remain sensitive to hysteresis, an averaging strategy is reported:
\[
\mathrm{LIR}_{\mathrm{avg}}=\frac{\mathrm{LIR}_{\mathrm{heating}}+\mathrm{LIR}_{\mathrm{cooling}}}{2}.
\]
Using this average can reduce \(S_T\) by approximately 50%, as demonstrated previously in the literature. The article further reports that co-doping trends are nontrivial: \(\Delta\mathrm{LIR}_1\) increases with Al\(^ {3+}\) or Sc\(^ {3+}\) individually, reaching about 0.8 at 10% Al\(^ {3+}\), but concurrent co-doping can mitigate uncertainty, with \(S_T \approx 3\) K for 10% Al\(^ {3+}\) and \(S_T \approx 2.2\) K for 2% Sc\(^ {3+}\).

## 5. Compositional phase-transition engineering

Phase-transition tuning is formalized through an ionic radius mismatch parameter,
\[
Q=\frac{R_{\mathrm{Ga}^{3+}}-R_{\mathrm{eff}}}{R_{\mathrm{Ga}^{3+}}},
\]
with
\[
R_{\mathrm{eff}}=(1-x_{\mathrm{Al}}-x_{\mathrm{Sc}})R_{\mathrm{Ga}^{3+}}+x_{\mathrm{Al}}R_{\mathrm{Al}^{3+}}+x_{\mathrm{Sc}}R_{\mathrm{Sc}^{3+}}.
\]
An empirical quadratic relation between \(T_c\) and \(Q\) is reported:
\[
T_c(Q)=28165.03\,Q^2-13501.21\,Q+433.71,
\]
where \(T\) is in K and \(Q\) is dimensionless. The quadratic fit is stated to outperform a linear model, although no goodness-of-fit metric such as \(R^2\) is reported [2507.13750].

The design logic is straightforward. Because Al\(^ {3+}\) is smaller than Ga\(^ {3+}\), substituting Al\(^ {3+}\) decreases \(R_{\mathrm{eff}}\), increases \(Q\), and lowers \(T_c\). Because Sc\(^ {3+}\) is larger than Ga\(^ {3+}\), substituting Sc\(^ {3+}\) increases \(R_{\mathrm{eff}}\), decreases \(Q\), and raises \(T_c\). The reported endpoint behavior follows this rule: 15% Al\(^ {3+}\) lowers the transition to 165 K in the abstract and approximately 180 K in the main text, while 2% Sc\(^ {3+}\) raises it to 491 K in the abstract and approximately 500 K in the main text.

The article also reports \(Q=0.02\) for 15% Al\(^ {3+}\) and \(Q=0.05\) for 2% Sc\(^ {3+}\), alongside extrapolations to approximately \(0.085\) at 100% Al and approximately \(-0.25\) at 100% Sc. Because the accompanying design discussion states that Sc\(^ {3+}\) decreases \(Q\), the quoted \(Q=0.05\) value for 2% Sc\(^ {3+}\) should be read as part of the reported dataset rather than as a fully resolved internal consistency statement. A plausible implication is that the qualitative tuning rule is more robust than any single tabulated intermediate value.

The resulting operating windows are broad and application-specific. The monotonic region of \(\mathrm{LIR}_1(T)\) extends from approximately 83 to 223 K for 15% Al\(^ {3+}\), and from approximately 83 to 553 K for 2% Sc\(^ {3+}\). This means that the same Eu\(^ {3+}\)-based readout can be adapted from sub-ambient operation to high-temperature process monitoring without substantial change in low-temperature spectral character.

## 6. Thermally activated emission, imaging, and experimental basis

The integrated Eu\(^ {3+}\) emission exhibits markedly different thermal quenching behavior depending on synthesis route. The Pechini sample falls to 25% of its initial intensity by approximately 400 K, whereas the solid-state sample maintains the 25% level up to approximately 575 K. The reported interpretation again emphasizes microstructure: smaller, more polydisperse grains correlate with earlier quenching and a broader phase transition, whereas larger, more uniform grains correlate with delayed quenching and a sharper transition [2507.13750].

Luminescence decays were fitted with a double exponential,
\[
I(t)=A_1\exp\!\left(-\frac{t}{\tau_1}\right)+A_2\exp\!\left(-\frac{t}{\tau_2}\right).
\]
No Arrhenius-type quenching parameters such as \(E_a\) or \(C\) were reported. The thermal evolution is instead described in terms of phase-transition-induced modification of radiative probabilities and nonradiative rates, consistent with the lifetime reduction from 1.06 ms to 0.81 ms.

Beyond point thermometry, the material supports digital-camera ratiometric thermal imaging. The green channel of a standard camera is dominated by the \(^{5}D_0 \rightarrow {}^{7}F_1\) band, while the red channel covers the full Eu\(^ {3+}\) emission; consequently, G/R image ratios track phase-transition-induced spectral changes. Under 254 nm UV excitation, thermally activated patterns were demonstrated from 453 K down to room temperature without additional optical filters, and phosphors with different \(T_c\) values were spatially arranged to produce anti-counterfeiting and thermal-mapping functions.

The experimental basis for these conclusions is methodologically explicit. Structural characterization used powder XRD on a PANalytical X’Pert Pro with Anton Paar HTK 1200N stage and Ni-filtered Cu K\(\alpha\) radiation at 40 kV and 30 mA over \(2\theta = 10\)–\(90^\circ\), referencing ICSD models 182539 and 182540. DSC used a PerkinElmer DSC 8000 with Controlled LN\(_2\) accessory at 20 K min\(^{-1}\). Microstructure was examined by FEI Nova NanoSEM 230 at 5.0 kV in beam deceleration mode, with image analysis in ImageJ and EDS at 30 kV using an EDAX Apollo X detector. Steady-state luminescence employed an FLS1000 spectrometer with 450 W Xe lamp and Hamamatsu R928 PMT; temperature control used a Linkam THMS600 with 0.1 K stability and 0.1 K resolution; decay measurements used a 150 W \(\mu\)Flash lamp; and imaging used a Canon EOS 400D with EFS 60 mm macro lens, with thermal verification by FLIR T540 at \(\pm 0.5\) K. The methods matter because the reported design rules rest on correlating spectroscopy, calorimetry, diffraction, and microstructure within a single material platform.

LaGaO\(_3\):Eu\(^ {3+}\) therefore occupies a distinctive position among phase-transition-based phosphors. Its performance is governed jointly by host-lattice symmetry breaking, Eu\(^ {3+}\) hypersensitive emission, grain-population statistics, and B-site ionic-radius engineering. In the reported implementation, these controls raise the relative sensitivity to \(18.2\%\ \mathrm{K}^{-1}\), reduce \(S_T\) to approximately 1.2 K, and tune the operating interval from approximately 83–223 K to approximately 83–553 K, while preserving low-temperature Eu\(^ {3+}\) spectral invariance across the co-doping series [2507.13750].

Source: https://www.emergentmind.com/topics/lagao3-eu3-phosphors