LaGaO3:Eu3+ Phosphors in Luminescence Thermometry

• LaGaO3:Eu3+ phosphors are luminescent materials that undergo a reversible phase transition, enabling ratiometric thermometry via Eu3+ spectral changes.
• The thermometric performance is governed by crystal chemistry tuning through B-site co-doping and microstructural control that sharpens phase transitions and reduces hysteresis.
• Advanced synthesis routes, including solid-state and Pechini methods, optimize grain-size dispersion and emission lifetimes for enhanced sensitivity and thermal response.

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+}$0 by Al$^ {3+}$1 and Sc$^ {3+}$2, thereby extending the usable temperature window while reducing hysteresis and increasing relative thermal sensitivity (Abbas et al., 18 Jul 2025).

1. Crystal chemistry and structural transition

LaGaO$^ {3+}$3 exhibits a reversible first-order phase transition from orthorhombic $^ {3+}$4 (space group No. 62; point symmetry $^ {3+}$5 at the La site) to rhombohedral $^ {3+}$6 (No. 167; point symmetry $^ {3+}$7 at the La site). In undoped material this occurs at approximately $^ {3+}$8–$^ {3+}$9 K. For LaGaO$^ {3+}$0:0.25%Eu$^ {3+}$1, differential scanning calorimetry gives $^ {3+}$2 K for the solid-state sample and a DSC peak at $^ {3+}$3 K for the Pechini-derived sample; the first-order nature is evidenced by hysteresis in both DSC and the luminescent intensity ratio (Abbas et al., 18 Jul 2025).

In this host, Eu$^ {3+}$4 substitutes La$^ {3+}$5 and is coordinated by $^ {3+}$6–$^ {3+}$7 O$^ {3+}$8 ions in distorted polyhedra, whereas Ga$^ {3+}$9 occupies octahedral sites with coordination number $^ {3+}$0. The co-dopants Al$^ {3+}$1 and Sc$^ {3+}$2 partially substitute Ga$^ {3+}$3, thus acting in the second coordination sphere relative to Eu$^ {3+}$4. The reported Shannon radii for coordination number $^ {3+}$5 are $^ {3+}$6 Å, $^ {3+}$7 Å, and $^ {3+}$8 Å. This site selectivity is central because it allows substantial modification of the host-lattice instability without directly altering the immediate Eu$^ {3+}$9 coordination shell.

The compositional range investigated comprises Al$^ {3+}$0 contents of $^ {3+}$1, $^ {3+}$2, and $^ {3+}$3 mol%, Sc$^ {3+}$4 content of $^ {3+}$5 mol%, and co-doped sets containing $^ {3+}$6 Al$^ {3+}$7 with $^ {3+}$8, $^ {3+}$9, or $_3$0 Sc$_3$1. Within this space, the transition temperature is shifted from $_3$2 K for 15% Al$_3$3 to $_3$4 K for 2% Sc$_3$5, while the main text also reports approximately $_3$6 K and $_3$7 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$8 spectroscopic center directly. In the reported system, the opposite conclusion is supported at low temperature: Al$_3$9/Sc$Pbnm$0 substitution is used precisely because it operates primarily through the second coordination sphere, decoupling phase-transition engineering from low-temperature Eu$Pbnm$1 local symmetry to first order.

2. Synthesis, phase purity, and microstructure

Two synthetic routes were compared. The solid-state route used La$Pbnm$2O$Pbnm$3 (99.999%), Ga$Pbnm$4O$Pbnm$5 (99.999%), Eu$Pbnm$6O$Pbnm$7 (99.99%), Al$Pbnm$8O$Pbnm$9 (99.995%), and Sc$R3c$0O$R3c$1 (99.99%), with dry mixing and grinding in hexane for 30 min, pre-annealing at 873 K for 3 h at 10 K min$R3c$2, regrinding, sintering at 1673 K for 6 h at 10 K min$R3c$3, and final grinding in air. The modified Pechini route used La$R3c$4O$R3c$5 and Eu$R3c$6O$R3c$7 converted to nitrates by dissolution in water with nitric acid and recrystallization, together with Ga(NO$R3c$8)$R3c$9H$^ {3+}$0O, 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 (Abbas et al., 18 Jul 2025).

Powder X-ray diffraction showed single-phase LaGaO$^ {3+}$1 for all examined co-doping levels, including 0.25% Eu$^ {3+}$2, up to 15% Al$^ {3+}$3, up to 2% Sc$^ {3+}$4, and their combinations. Peak shifts track the expected unit-cell contraction for Al$^ {3+}$5 substitution and expansion for Sc$^ {3+}$6 substitution. For 10% and 15% Al$^ {3+}$7, split reflections near $^ {3+}$8 indicate partial stabilization of the high-temperature $^ {3+}$9 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$^{5}D_0 \rightarrow {}^{7}F_J$0:Eu$^{5}D_0 \rightarrow {}^{7}F_J$1: thermometric performance is not solely dictated by crystal chemistry. It is also controlled by mesoscale population statistics, especially grain-size dispersion.

3. Eu$^{5}D_0 \rightarrow {}^{7}F_J$2 luminescence and ratiometric observables

The Eu$^{5}D_0 \rightarrow {}^{7}F_J$3 emission spectrum is dominated by $^{5}D_0 \rightarrow {}^{7}F_J$4 transitions with bands centered at approximately 575 nm ($^{5}D_0 \rightarrow {}^{7}F_J$5), 590 nm ($^{5}D_0 \rightarrow {}^{7}F_J$6), 620 nm ($^{5}D_0 \rightarrow {}^{7}F_J$7), 650 nm ($^{5}D_0 \rightarrow {}^{7}F_J$8), and 700 nm ($^{5}D_0 \rightarrow {}^{7}F_J$9). The $^ {3+}$00 band is characterized as magnetic dipole and symmetry-insensitive, whereas $^ {3+}$01 is electric dipole and hypersensitive to local symmetry (Abbas et al., 18 Jul 2025).

Across the orthorhombic-to-rhombohedral transition, the relative intensity of $^ {3+}$02 increases with respect to $^ {3+}$03, while the $^ {3+}$04 intensity decreases relative to $^ {3+}$05. Simultaneously, the Stark structure simplifies in the high-temperature phase, consistent with the increase in site symmetry from $^ {3+}$06 to $^ {3+}$07. Peaks shift modestly and band shapes change. The $^ {3+}$08 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,

$^ {3+}$09

implemented in practice through selected Stark components in wavelength windows around the $^ {3+}$10 and $^ {3+}$11 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+}$12 site at low temperature: $^ {3+}$13 Across the Al$^ {3+}$14/Sc$^ {3+}$15 series, $^ {3+}$16 at 83 K. Low-temperature decay profiles at 83 K are likewise nearly identical among compositions. Together, these observations support the claim that co-doping shifts the host transition while leaving the low-temperature Eu$^ {3+}$17 spectroscopic environment essentially unchanged.

4. Thermometric performance and hysteresis

The relative thermal sensitivity is defined as

$^ {3+}$18

For LaGaO$^ {3+}$19:0.25%Eu$^ {3+}$20, the Pechini sample reaches $^ {3+}$21 at approximately 460 K, whereas the solid-state sample reaches $^ {3+}$22 at approximately 420 K. The abrupt increase in $^ {3+}$23 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 (Abbas et al., 18 Jul 2025).

The thermometric limitation intrinsic to a first-order transition is hysteresis. It was quantified using

$^ {3+}$24

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 $^ {3+}$25, 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 $^ {3+}$26, while the narrower transition distribution reduces loop width.

For applications that remain sensitive to hysteresis, an averaging strategy is reported: $^ {3+}$27 Using this average can reduce $^ {3+}$28 by approximately 50%, as demonstrated previously in the literature. The article further reports that co-doping trends are nontrivial: $^ {3+}$29 increases with Al$^ {3+}$30 or Sc$^ {3+}$31 individually, reaching about 0.8 at 10% Al$^ {3+}$32, but concurrent co-doping can mitigate uncertainty, with $^ {3+}$33 K for 10% Al$^ {3+}$34 and $^ {3+}$35 K for 2% Sc$^ {3+}$36.

5. Compositional phase-transition engineering

Phase-transition tuning is formalized through an ionic radius mismatch parameter,

$^ {3+}$37

with

$^ {3+}$38

An empirical quadratic relation between $^ {3+}$39 and $^ {3+}$40 is reported: $^ {3+}$41 where $^ {3+}$42 is in K and $^ {3+}$43 is dimensionless. The quadratic fit is stated to outperform a linear model, although no goodness-of-fit metric such as $^ {3+}$44 is reported (Abbas et al., 18 Jul 2025).

The design logic is straightforward. Because Al$^ {3+}$45 is smaller than Ga$^ {3+}$46, substituting Al$^ {3+}$47 decreases $^ {3+}$48, increases $^ {3+}$49, and lowers $^ {3+}$50. Because Sc$^ {3+}$51 is larger than Ga$^ {3+}$52, substituting Sc$^ {3+}$53 increases $^ {3+}$54, decreases $^ {3+}$55, and raises $^ {3+}$56. The reported endpoint behavior follows this rule: 15% Al$^ {3+}$57 lowers the transition to 165 K in the abstract and approximately 180 K in the main text, while 2% Sc$^ {3+}$58 raises it to 491 K in the abstract and approximately 500 K in the main text.

The article also reports $^ {3+}$59 for 15% Al$^ {3+}$60 and $^ {3+}$61 for 2% Sc$^ {3+}$62, alongside extrapolations to approximately $^ {3+}$63 at 100% Al and approximately $^ {3+}$64 at 100% Sc. Because the accompanying design discussion states that Sc$^ {3+}$65 decreases $^ {3+}$66, the quoted $^ {3+}$67 value for 2% Sc$^ {3+}$68 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 $^ {3+}$69 extends from approximately 83 to 223 K for 15% Al$^ {3+}$70, and from approximately 83 to 553 K for 2% Sc$^ {3+}$71. This means that the same Eu$^ {3+}$72-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+}$73 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 (Abbas et al., 18 Jul 2025).

Luminescence decays were fitted with a double exponential,

$^ {3+}$74

No Arrhenius-type quenching parameters such as $^ {3+}$75 or $^ {3+}$76 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 $^ {3+}$77 band, while the red channel covers the full Eu$^ {3+}$78 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 $^ {3+}$79 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$^ {3+}$80 radiation at 40 kV and 30 mA over $^ {3+}$81–$^ {3+}$82, referencing ICSD models 182539 and 182540. DSC used a PerkinElmer DSC 8000 with Controlled LN$^ {3+}$83 accessory at 20 K min$^ {3+}$84. 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 $^ {3+}$85Flash lamp; and imaging used a Canon EOS 400D with EFS 60 mm macro lens, with thermal verification by FLIR T540 at $^ {3+}$86 K. The methods matter because the reported design rules rest on correlating spectroscopy, calorimetry, diffraction, and microstructure within a single material platform.

LaGaO$^ {3+}$87:Eu$^ {3+}$88 therefore occupies a distinctive position among phase-transition-based phosphors. Its performance is governed jointly by host-lattice symmetry breaking, Eu$^ {3+}$89 hypersensitive emission, grain-population statistics, and B-site ionic-radius engineering. In the reported implementation, these controls raise the relative sensitivity to $^ {3+}$90, reduce $^ {3+}$91 to approximately 1.2 K, and tune the operating interval from approximately 83–223 K to approximately 83–553 K, while preserving low-temperature Eu$^ {3+}$92 spectral invariance across the co-doping series (Abbas et al., 18 Jul 2025).