Electronic absorption spectra of vanadium-doped zircon ⁴

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Vanadium doped zircon (ZrSiO₄:V) shows a deep blue colour and provides a superior thermal and chemical stability making it an important industrial pigment. The colour is given by the following absorption spectrum:

Due to the low level of doping it is not possible to extract the crystal structure by indirect spectroscopic methods. It remains the question: What is the chromophore and what is the location of vanadium in the zircon host lattice? The zircon crystal structure is known as:

view on the (001) plane		view on the (111) plane

The label "Int" marks an interstitial site the zircon crystal structure provides. Finally there are three possible locations for the vanadium:

1. Replace zircon
2. Replace silicon
3. Reside at the insterstitial site

Ligand-Field Theory

• V electron configuration: 1s²2s²2p⁶3s²3p⁶3d⁵
• V⁴⁺ electron configuration: 1s²2s²2p⁶3s²3p⁶3d¹ $\Longrightarrow$ 3d¹ ion
• "Standard"-splitting:
  
• Allowed electronical transitions:

  Site	Symmetry	# transitions
  V@Zr, V@Si	D2d	one ore two *
  V@Int	C2	four

  ^* depending on ground state

Quantum Chemical Calculation

• V⁴⁺ ion in Madelung Potential
  • Approximation by about 500 point charges located at the regular places of the ions
  • Least square fit
  • Optimization of charges within a sphere of 4.5 Å radius around the vanadium atom.

Madelung potential at V⁴⁺

Simulated spectrum

Expected excitations

		f /10-8
State	$\Delta$E / eV 5	x = y	z
X 2A1	0.00	--	--
1 2E	0.04	0.04	--
1 2B1	0.05	--	--
1 2B2	0.59	--	1.3

		f /10-8
State	$\Delta$E / eV 6	x = y	z
X 2B1	0.00	--	--
1 2A1	0.11	--	--
1 2B2	0.17	--	--
1 2E	0.47	2.4	--

		f /10-7
State	$\Delta$E / eV 7	x	y	z
X 2A	0.00	--	--	--
1 2B	0.86	--	2.3	6.3
1 2A	1.23	2.0	--	--
2 2B	1.27	--	0.2	4.2
2 2A	1.50	27	--	--

The crystal field at the zircon and silicon sites is highly spherically symmetric and there is only little energy splitting of the d-orbitals. For the interstitial site however the field is highly non-symmetric yielding one energy lowered d-orbital. Consequently the excitation energies are largest for this case. To summerize: The simultated spectra strongly suggest that the observed absorption spectrum is caused by vanadium ions located at the interstitial sites [13].

Footnotes

... zircon⁴

In cooperation with A. Niesert, Institute for Anorganic Chemistry, University of Bonn and M. Jansen, MPI for Solid State Research, Stuttgart and S. Siggel, TU Berlin, Institute for Applied Geology, Berlin

... eV⁵

at enlargement factor 0.88, corresponding to expected collapse of the Zr-site due to different ionic radii

... eV⁶

at enlargement factor 1.08, corresponding to expected expansion of the Si-site due to different ionic radii

... eV⁷

at enlargement factor 1, corresponding to EXAFS results from S. Siggel