I am a phd student, may I buy a ipad with educational discount

Education Pricing and Student Discounts - Apple

I don't know. You need to read the requirements to find out if your institution qualifies.

Education - How to Buy - Apple

Its not that much, it may be better to look out for deals from a third party. Or buy the refurbished ones from the online apple store.

for example new ipad 2018 $329.

Educational Price $300