my answer is as below which can be wrong but might be helpful.

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the Key Components

First, let’s define the components involved:

• Host Lattice: Calcium Molybdate (CaMoO₄)CaMoO₄ has a scheelite crystal structure. The fundamental building block responsible for its optical properties is the molybdate group, (MoO₄)²⁻. This is a tetrahedron where a central Molybdenum ion (Mo⁶⁺) is surrounded by four Oxygen ions (O²⁻).
• Photoluminescence Excitation (PLE) SpectrumA PLE spectrum is obtained by monitoring the emission intensity at a specific wavelength (e.g., a characteristic emission of Dy³⁺) while scanning the excitation wavelength. A peak in the PLE spectrum indicates a wavelength at which the material strongly absorbs light, which is then efficiently transferred to the emitting center (Dy³⁺).
• The Charge Transfer (CT) Band in Undoped CaMoO₄The broad, intense absorption band observed in the UV region (typically around 250-320 nm) for pure CaMoO₄ is not a transition within a single ion. It is a Charge Transfer (CT) transition, specifically from the filled 2p orbitals of the oxygen ligands (O²⁻) to the empty 4d orbitals of the central molybdenum ion (Mo⁶⁺). In simple terms: O²⁻ → Mo⁶⁺ Charge Transfer. This process effectively creates a short-lived [MoO₄]³⁻ complex. The energy of this transition is highly sensitive to the local environment, particularly the Mo-O bond length and the electronic environment around the molybdate group.

The Effect of Dy³⁺ Doping

When you introduce Dysprosium (Dy³⁺) ions into the CaMoO₄ lattice, they act as a perturbation.

• Substitution Site: Due to their similar ionic radii and charge preferences, trivalent rare-earth ions like Dy³⁺ preferentially substitute for the divalent Calcium (Ca²⁺) ions.Ionic Radius (for 8-coordination): Ca²⁺ ≈ 1.12 Å | Dy³⁺ ≈ 1.05 Å. The Dy³⁺ ion is slightly smaller than the Ca²⁺ ion it replaces.
• The Core Problem: Charge MismatchThis substitution is not perfect. You are replacing a +2 ion (Ca²⁺) with a +3 ion (Dy³⁺). This creates a net positive charge in the lattice at the site of substitution, denoted as DyCa∙​ in Kröger-Vink notation. To maintain overall electrical neutrality, the crystal lattice must create compensating defects. The most common mechanism in this system is the formation of Calcium vacancies (VCa′′). This means for every two Dy³⁺ ions that substitute for Ca²⁺, one calcium vacancy is created.

The Mechanisms Causing the Blue Shift

The blue shift of the O²⁻ → Mo⁶⁺ CT band occurs because the doping process increases the energy required for this charge transfer to happen. As you increase the Dy³⁺ concentration, these effects become more pronounced. Here are the primary reasons:

Mechanism 1: Lattice Contraction and Mo-O Bond Covalency (The Primary Cause)

This is the most significant factor.

Analogy: Imagine trying to pull a ball away from a magnet. If you replace the original magnet with a stronger one, you need to use more energy to pull the ball away by the same distance. Here, the O²⁻ electron is the ball, and the Mo-O atomic system is the magnet, which is “strengthened” by the influence of the nearby Dy³⁺.

In the diagram:

• Left: The CT energy gap (ΔE1​) in the unperturbed CaMoO₄ lattice.
• Right: The Dy³⁺ ion lowers the energy of the O 2p ground state. This results in a larger energy gap (ΔE2>ΔE1), which corresponds to a shorter wavelength (λ2
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