Mdthbs

For an application I need a commercial, transparent liquid with refractive index $n>2$.

1. Is this kind of liquid available?




2. What's the chemical composition of this?

On the web, only $n=1.81$ is routine.

Definition of refractive index:

$n=frac {c}{v}$

If n>2 then this means that the velocity of light in the material is less than half than in vacuum so there must be a strong interaction with the material.

One of the highest refractory index in liquids is 1.79(20°C) possessed by a solution of sulfur in methylene iodide($CH_2I_2$). Liquids are less dense than solids and this explains why it's difficult to have a refractory index greater than 2.
I found a nature article that says:

Selenium monobromide, Se2Br2, has a higher
refractive index than that of any pure liquid hitherto
recorded. Prepared by direct combination, the value
for $nu$ is 1.96±0.01 rising to 2.02 on exposure to the
atmosphere, owing to decomposition of the bromide,
with separation and reabsorption of selenium.

https://www.nature.com/articles/133066b0

EDIT: From a more recent paper(2016), selenium monobromide is confirmed as one of the liquid with the highest refraction index(>2). However, diiodomethane is more easily accessible commercially.

A promising candidate solvent is the organic liquid diiodomethane (CH2I2), which is one of the liquids with the highest
known refractive index values (n = 1.74). While other high
refractive index liquids exist (phenyldi-iodoarsine (C6H5AsI2)
with n = 1.85 and selenium monobromide (Se2Br2) with n=
2.1), diiodomethane has the key advantage of being commercially available. In addition, diiodomethane is an excellent
solvent, and many liquid formulations using salts dissolved in
diiodomethane are reported to increase the refractive index
and are even available commercially.

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-55-12-3165

To complement @blu_potatos's answer, there are some other contenders having refractive index greater than 2

1. 
   Arsenic di/trisulfide and sulfur and/or selenium and/or mercuric sulfide in arsenic tribromide solvent having refractive index ranging from 2.0 to 2.07 (arsenic tribromide seem to attack the lead-glass prisms of refractometers, corroding metal, poisonous in nature and reactive toward some minerals)


2. 
   phosphorus and/or sulfur in carbon sulfide solvent having R.I= 2.01/2.07 (carbon disulfide is highly volatile, very flammable and poisonous.
   Solutions containing white phosphorus should be kept in the dark as
   light causes the conversion of the white phosphorus to the red form,
   which is more poisonous).


3. 
   Selenium and/or sulfur in phosphorus solvent having R.I =2.2-2.5 (Solutions of selenium in phosphorus cannot be stored for long under water as they decompose).


4. 
   Selenium in selenium monobromide solvent having R.I>2.02 (Selenium monobromide is unstable decomposing to selenium)


5. 
   A ternary system of phosphorus-sulfur-methylene iodide having R.I upto 2.06 has been observed

begin{array}{c|c}
mathbf{component} & mathbf{R.I} \hline
text{P(solid) at 29°C } & text{2.15}\
text{P(liq.) at 44°C } & text{2.10}\
text{P-S-CH2I2 at 25°C } & text{2.06} \
text{P-CH2I2(satd.) at 18°C} & text{1.94}\
text{CH2I2 at 15°C } & text{1.74}end{array}

There are loads of other liquids having R.I value in between 1.5-2.0 which are stable and can be used commercially. See references

References

1. 
   http://www.minsocam.org/ammin/AM40/AM40_398.pdf (1-4)


2. 
   http://www.minsocam.org/ammin/AM21/AM21_245.pdf (5)


3. https://www.osapublishing.org/DirectPDFAccess/71A8545D-AFC6-6350-82F044C815273272_338928/ao-55-12-3165.pdf?da=1&id=338928&seq=0&mobile=no

Solutions of phosphorous in carbon disulfide are not only smelly and toxic, but they're hideously prone to catching fire. Methyene iodide is carcinogenic. Selenium and its compounds are toxic and notoriously evil-smelling.
Because we're playing around in the periodic table's bad neighborhood, high RI liquids are uniformly, extremely unpleasant.

The refractive index of liquid iodine is 1.934 at 114°C
https://www.tandfonline.com/doi/abs/10.1080/13642818608239022
This might be the best candidate for conducting experiments. Transparency in the visible range (above 450 nm) is very limited, but at least the stuff won't incinerate you, kill you, or induce your neighbors to kill you.

Based on the OP’s two questions and the OP’s several comments, the OP wants to know if there is a commercially available liquid which, at 23°C, is transparent in the range 380 to 700 nm. As the OP stated in a comment, the liquid should be “not poisonous and can be used as gap fillers in refractometers (gap between prism and stones).”

In gemological testing of the refractive indices of gemstones, the upper limit of refractive index is 1.81, if a contact liquid is used in a conventional gemological refractometer. See, for example, https://www.gemsociety.org/article/measuring-gemstone-ri-1-81/ . As noted in this substantive article, at http://gemologyproject.com/wiki/index.php?title=Refractometer , the two high refractive index contact liquids are

1. a saturated solution of sulfur in di-iodomethane (n = 1.79) and


2. a saturated solution of sulfur, di-iodomethane and tetraiodoethylene (n = 1.81).

Not the most pleasant solutions, but much better than the exotic and far more toxic possibilities that “are so toxic that they are only used in specially equipped laboratories”. (Quote from http://gemologyproject.com/wiki/index.php?title=Refractometer).

So the answers to the OP's two questions are "no" and "does not apply", respectively.