Optical Properties and Growth Mechanism of Macroporous La₃Ga₅.₅Nb₀.₅O₁₄ Ceramic Synthesized via Pechini Process

Abstract

We investigated the optical characteristics and growth pathway of La₃Ga_5.5Nb_0.5O₁₄ (LGN) powders produced by the Pechini sol‑gel method. After calcination at 600–800 °C, X‑ray diffraction confirmed the emergence of a single‑phase orthorhombic LGN structure (JCPDS 47‑0533) at 800 °C. A schematic growth mechanism is proposed, detailing the transformation of precursor nano‑foams into macroporous crystalline grains. Photoluminescence measurements, excited at 327 nm, reveal a broad blue emission centered at 475 nm (77 K), attributed to the [NbO₆]^7– octahedral group. The 800 °C sample exhibits a sharp optical absorption edge at 320 nm, corresponding to a band‑gap energy of 3.95 eV and indicating minimal oxygen‑vacancy defects.

Introduction

Compounds with the Ca₃Ga₂Ge₄O₁₄ (CGG) structure—such as La₃Ga₅SiO₁₄ (LGS), La₃Ga_5.5Nb_0.5O₁₄ (LGN), and La₃Ga_5.5Ta_0.5O₁₄ (LGT)—have been extensively explored for their piezoelectric and optical properties. These materials underpin high‑performance bulk acoustic wave (BAW) and surface acoustic wave (SAW) filters, oscillators, and nonlinear optical devices. LGN, in particular, exhibits a wide mid‑infrared transparency window (2–6 µm) and has been characterized for phase‑matching in second‑harmonic and difference‑frequency generation up to 6.5 µm. Recent studies have demonstrated Nd‑doped LGN lasers with tunable output under diode pumping.

Macroporous ceramics, defined by porosities of 5–90 % and pore sizes exceeding 100 nm, combine high mechanical strength, thermal conductivity, and chemical stability, making them attractive for filtration, catalysis, thermoelectric conversion, and biomedical applications. Their photonic band gaps enable advanced uses in optical communications and gas sensing. However, LGN‑based macroporous ceramics prepared via chemical routes remain underexplored. Traditional synthesis of LGN single crystals requires temperatures above 1500 °C (Czochralski method), whereas the Pechini process offers a low‑temperature alternative (~800 °C) with superior compositional homogeneity and particle size control.

In this work, we employ the Pechini method to synthesize a single‑phase, orthorhombic LGN ceramic and systematically investigate its growth mechanism, morphology, and optical properties.

Methods/Experimental

Materials Used

High‑purity lanthanum nitrate hexahydrate La(NO₃)₃·6H₂O, gallium nitrate Ga(NO₃)₃, niobium pentachloride NbCl₅, citric acid anhydrous (CA), and ethylene glycol (EG) were used.

Preparation of La₃Ga_5.5Nb_0.5O₁₄

The LGN precursor was prepared by dissolving stoichiometric amounts of La(NO₃)₃, Ga(NO₃)₃, and NbCl₅ in water. NbCl₅ reacts with ethanol to form niobium ethoxide (Nb(OC₂H₅)₅) according to:

\[\mathrm{NbCl}_5 + 5\mathrm{C}_2\mathrm{H}_5\mathrm{OH} \rightarrow \mathrm{Nb}(\mathrm{OC}_2\mathrm{H}_5)_5 + 5\mathrm{HCl}\]

A chelating agent (CA) was added in a 2:1 molar ratio to the metal ions, followed by ethylene glycol to promote polymerization. The resulting gel was dried at 120 °C for 24 h and then sintered in air at 600–800 °C for 3 h to obtain the macroporous LGN ceramic.

Characterization/Phase Identification

DTA–TGA (PE–DMA 7) monitored the thermal decomposition of the precursor. X‑ray powder diffraction (Rigaku Dmax‑33) identified the crystal phases. High‑resolution TEM (HF‑2000, Hitachi) examined morphology and microstructure. Photoluminescence (Hitachi‑4500, xenon lamp) measured excitation/emission spectra at 300 K and 77 K. UV–vis absorption (Hitachi U‑3010) evaluated the band‑gap energy.

Results and Discussion

The amorphous precursor undergoes pyrolysis to form crystalline LGN. The overall decomposition reaction can be summarized as:

\[3\mathrm{La(NO}_3)_3 + 5.5\mathrm{Ga(NO}_3)_2 + 0.5\mathrm{Nb(OC}_2\mathrm{H}_5)_5 \xrightarrow{\text{CA}} \mathrm{La}_3\mathrm{Ga}_{5.5}\mathrm{Nb}_{0.5}\mathrm{O}_{14} + \mathrm{NO}_2\uparrow + \mathrm{H}_2\mathrm{O}\uparrow + \mathrm{CO}_2\uparrow + \mathrm{C}_2\mathrm{H}_5\mathrm{OH}\uparrow\]

Figure 1 shows XRD patterns at 600, 700, and 800 °C. At 600 °C, weak micro‑crystalline peaks appear; by 700 °C, crystallization initiates; at 800 °C, a sharp, single‑phase orthorhombic LGN pattern (JCPDS 47‑0533) emerges, confirming full crystallization.

Optical Properties and Growth Mechanism of Macroporous La₃Ga₅.₅Nb₀.₅O₁₄ Ceramic Synthesized via Pechini Process

FT‑IR spectra (Figure 2) reveal strong CO₂ stretching at 2300 nm and nitrate absorption at 1500 nm at 600–700 °C. New peaks between 500–600 nm appear at 800 °C, indicating the formation of LGN nanocrystals and the presence of residual organics.

TEM images (Figure 3) illustrate the precursor morphology at 600 °C: nano‑foam structures with diameters <100 nm are evident. As the temperature rises, these foams collapse, creating microporous voids and promoting grain growth. Higher‑magnification TEM (Figure 4) confirms the emergence of nanocrystalline domains.

A schematic growth mechanism (Figure 6) describes the evolution from nano‑foams at 600 °C to macroporous grains at 800 °C. During 600–700 °C, foams expand and collapse, forming pores <100 nm. Concurrently, micro‑crystals nucleate and coalesce, reducing interfacial energy and yielding well‑defined macroporous grains.

Photoluminescence spectra (Figure 7) show a prominent blue emission at 475 nm under 327 nm excitation at 77 K, attributable to the [NbO₆]^7– octahedral group. At 300 K, the emission intensity diminishes due to thermal quenching via non‑radiative phonon coupling and defect trapping.

UV‑vis absorption (Figure 8) reveals an onset at 320 nm, corresponding to a band‑gap energy of 3.95 eV. Minor absorption bumps near 320 nm at lower temperatures indicate oxygen‑vacancy defects, which diminish as the sintering temperature increases.

Conclusions

We successfully synthesized macroporous LGN ceramics via the Pechini route, achieving a single‑phase orthorhombic structure at 800 °C. The material exhibits a 327 nm excitation band arising from Nb⁵⁺–O^2– charge transfer and a strong blue photoluminescence at 475 nm (77 K) linked to [NbO₆]^7–. The 800 °C sample shows a clear absorption edge at 320 nm, confirming a band gap of 3.95 eV and indicating minimal oxygen vacancies. The proposed growth mechanism highlights the transformation of nano‑foams into macroporous grains during sintering.