Sb-Doped Cs₂TiCl₆: Structure & Optoelectronics

• Sb-doped Cs₂TiCl₆ is a vacancy-ordered double perovskite where minor Sb³⁺ substitution alters lattice dynamics and suppresses impurity phases within a cubic Fm̅3m structure.
• The aliovalent doping induces a low-temperature order–disorder transition with a new Raman mode near 314–319 cm⁻¹, reflecting enhanced electron–phonon coupling.
• Enhanced self-trapped exciton emission and robust high-pressure stability highlight this material’s promise for next-generation lead-free optoelectronic devices.

Sb-doped Cs$_2$TiCl$_6$ is a vacancy-ordered double perovskite in which a few percent of the Ti$^{4+}$ cation sites are substituted with Sb$^{3+}$. This aliovalent site doping induces significant modifications to the crystal's structural, vibrational, and optoelectronic properties while preserving the robust cubic Fm$\bar{3}$m lattice symmetry. Sb doping critically reduces impurity phases, modulates low-temperature phase behavior through a novel order–disorder transition, enhances electron–phonon coupling, and imparts structural stability under high pressure, making it a promising platform for the design of advanced, lead-free optoelectronic materials (Tiwari et al., 26 Dec 2025).

1. Synthesis, Structure, and Doping Chemistry

Sb-doped Cs$_2$TiCl$_6$ is synthesized via an acid-assisted precipitation reaction in which CsCl is dissolved in concentrated HCl at 60 °C, and either TiCl$_4$ or a mixture of TiCl$_4$ and SbCl$_3$ (providing 2–3% Sb relative to Ti) is added. Subsequent washing, centrifugation, and drying yield phase-pure powders. Powder X-ray diffraction reveals all samples retain the cubic Fm$\bar{3}$m A$_2$BX$_6$ lattice, with only minor shifts (~1° to higher 2θ) in peak positions upon Sb incorporation, consistent with a decrease in lattice constant resulting from the shorter Sb–Cl bond length.

Structurally, this perovskite framework consists of a network of isolated [BX$_6$]$^{2-}$ octahedra in which B-site cations (Ti$^{4+}$, or substituted Sb$^{3+}$) and vacancies alternate in a 1:1 order. Sb$^{3+}$ occupies Ti$^{4+}$ sites stochastically; charge compensation is provided by the intrinsic vacancy order. The overall cubic symmetry is retained in the doped materials at room temperature.

Synthesis Step	Reagents & Conditions	Purpose
CsCl dissolved	2 mL conc. HCl @ 60°C	Source of Cs$^+$, Cl$^-$
TiCl$_4$/SbCl$_3$ added	molar % Sb = 2–3	Aliovalent doping (Sb$^{3+}$/Ti$^{4+}$ replacement)
IPA wash, centrifuge, dry	7000 rpm, 60°C	Purification and powder formation

2. Phase Purity Enhancement and Structural Model

Raman spectroscopy at 300 K allows quantitative assessment of phase purity. Pristine and Bi-doped Cs$_2$TiCl$_6$ exhibit, in addition to the three fundamental [TiCl$_6$]$^{2-}$ octahedral vibrational modes, extraneous peaks (~120, 256, 270 cm$^{-1}$) attributable to impurity phases such as unreacted precursors. In contrast, Sb-doped samples (2% and 3%) display exclusively the three intrinsic vibrational modes, confirming that Sb$^{3+}$ preferentially promotes clean crystallization and dramatically suppresses impurity contributions. The cubic network of isolated [BX$_6$]$^{2-}$ octahedra remains globally intact, and the local environment is determined by the presence or absence of Sb$^{3+}$ at the B-site.

3. Vibrational Properties and Temperature-Dependent Raman Spectroscopy

At room temperature, the vibrational spectrum of Sb-doped Cs$_2$TiCl$_6$ comprises only the expected T$_{2g}$ (Cs$^{+}$ translation, 50–52 cm$^{-1}$), T$_{2g}$ (Ti–Cl bending, 179–182 cm$^{-1}$), and A$_{1g}$ (Ti–Cl stretching, 312–318 cm$^{-1}$) modes. Upon cooling from 273 K to 4 K, these modes harden and reduce in width (reflecting suppressed anharmonicity), but in Sb-doped samples a sharp anomaly occurs at ~100 K:

• A new Raman mode (M$_1$) appears at 314–319 cm$^{-1}$, adjacent to the A$_{1g}$ stretch.
• Both the first-order temperature coefficient ($\chi$) and anharmonic constant ($A$) change by factors of 2–8×.
• Fitting the frequencies and linewidths with models combining linear (Grüneisen) and three-phonon decay terms produces distinct parameter sets above and below 100 K, e.g. for 2% Sb-doped:
  • $\chi$(T < 100 K) ≃ –2.43 × 10$^{-3}$ cm$^{-1}$ K$^{-1}$, $A$(T < 100 K) ≃ 0.423 cm$^{-1}$
  • $\chi$(T > 100 K) ≃ –20.23 × 10$^{-3}$ cm$^{-1}$ K$^{-1}$, $A$(T > 100 K) ≃ 0.013 cm$^{-1}$

No such transition is observed in pristine Cs$_2$TiCl$_6$, where all mode parameters change smoothly with temperature.

4. Low-Temperature Order–Disorder Transition

The combination of a new M$_1$ mode, abrupt changes in vibrational parameters, and mode sharpening at 100 K in Sb-doped samples indicates a partial order–disorder transition absent from the undoped material. This transformation is likely due to the freezing-in of local Sb–Cl coordination environments or a reorganization of dynamic vacancy ordering, reflecting the local symmetry-breaking effects of aliovalent Sb$^{3+}$. The cubic global symmetry persists (no mode splitting), suggesting the structural change is local rather than long-range. The distinct trivalent chemistry and ionic size of Sb$^{3+}$ compared to Ti$^{4+}$ play crucial roles in driving this phenomenon.

5. Optoelectronic Properties and Electron–Phonon Coupling

Photoluminescence measurements under 375 nm excitation reveal that all Sb-doped samples emit broad spectra centered at 448 nm, characteristic of self-trapped excitons associated with isolated [BX$_6$]$^{2-}$ octahedra. The full width at half maximum (FWHM) of the PL increases substantially in 2% Sb-doped samples (164.73 nm) relative to Bi-doped analogues (138.2 nm), a quantitative manifestation of stronger electron–phonon coupling and greater phonon anharmonicity in the Sb-doped lattice. This optical response aligns with the larger anharmonic constants ($A$) extracted from low-temperature Raman fits, confirming the critical role of Sb doping in modulating lattice dynamics and carrier localization.

6. High-Pressure Structural Robustness

When subject to pressures up to 30 GPa in a diamond anvil cell, Sb-doped Cs$_2$TiCl$_6$ retains its cubic structural motif. Both T$_{2g}$ and A$_{1g}$ Raman modes shift monotonically to higher frequencies (dω/dP ≃ +2–3 cm$^{-1}$/GPa) with increasing pressure, and the PL emission exhibits a continuous redshift, indicating compression of self-trapped exciton states. No phase transitions, mode splitting, or new features are observed throughout the pressure range studied, demonstrating exceptional mechanical and structural stability.

7. Implications and Outlook for Optoelectronic Device Engineering

Strategic Sb doping in Cs$_2$TiCl$_6$ enables a twofold functional enhancement: pronounced suppression of impurity phases leading to cleaner vibrational and photonic characteristics, and tunability of lattice anharmonicity as well as low-temperature phase behavior via an order–disorder transformation near 100 K. Enhanced self-trapped exciton emission and robust stability under mechanical stress highlight the material's promise for broadband light emitters and scintillator applications. More broadly, the ability to control octahedral confinement, phonon interactions, and phase phenomena in vacancy-ordered, lead-free perovskites via aliovalent doping outlines a pathway for creating resilient, tunable materials for next-generation optoelectronic devices (Tiwari et al., 26 Dec 2025).